diff --git a/hibp.go b/hibp.go
index 5dd01b9..57f1beb 100644
--- a/hibp.go
+++ b/hibp.go
@@ -49,8 +49,18 @@ var (
 	// expected length
 	ErrSHA1LengthMismatch = errors.New("SHA1 hash size needs to be 160 bits")
 
+	// ErrNTLMLengthMismatch should be used if a given NTLM hash does not match the
+	// expected length
+	ErrNTLMLengthMismatch = errors.New("NTLM hash size needs to be 128 bits")
+
 	// ErrSHA1Invalid should be used if a given string does not represent a valid SHA1 hash
 	ErrSHA1Invalid = errors.New("not a valid SHA1 hash")
+
+	// ErrNTLMInvalid should be used if a given string does not represent a valid NTLM hash
+	ErrNTLMInvalid = errors.New("not a valid NTLM hash")
+
+	// ErrUnsupportedHashMode should be used if a given hash mode is not supported
+	ErrUnsupportedHashMode = errors.New("hash mode not supported")
 )
 
 // Client is the HIBP client object
@@ -80,7 +90,10 @@ func New(options ...Option) Client {
 
 	// Set defaults
 	c.to = DefaultTimeout
-	c.PwnedPassAPIOpts = &PwnedPasswordOptions{}
+	c.PwnedPassAPIOpts = &PwnedPasswordOptions{
+		HashMode:    HashModeSHA1,
+		WithPadding: false,
+	}
 	c.ua = DefaultUserAgent
 
 	// Set additional options
@@ -95,7 +108,10 @@ func New(options ...Option) Client {
 	c.hc = httpClient(c.to)
 
 	// Associate the different HIBP service APIs with the Client
-	c.PwnedPassAPI = &PwnedPassAPI{hibp: &c}
+	c.PwnedPassAPI = &PwnedPassAPI{
+		hibp:     &c,
+		ParamMap: make(map[string]string),
+	}
 	c.BreachAPI = &BreachAPI{hibp: &c}
 	c.PasteAPI = &PasteAPI{hibp: &c}
 
@@ -140,6 +156,18 @@ func WithRateLimitSleep() Option {
 	}
 }
 
+// WithPwnedNTLMHash sets the hash mode for the PwnedPasswords API to NTLM hashes
+//
+// Note: This option only affects the generic methods like PwnedPassAPI.CheckPassword
+// or PwnedPassAPI.ListHashesPassword. For any specifc method with the hash type in
+// the method name, this option is ignored and the hash type of the function is
+// forced
+func WithPwnedNTLMHash() Option {
+	return func(c *Client) {
+		c.PwnedPassAPIOpts.HashMode = HashModeNTLM
+	}
+}
+
 // HTTPReq performs an HTTP request to the corresponding API
 func (c *Client) HTTPReq(m, p string, q map[string]string) (*http.Request, error) {
 	u, err := url.Parse(p)
diff --git a/hibp_test.go b/hibp_test.go
index 2d81e28..7389172 100644
--- a/hibp_test.go
+++ b/hibp_test.go
@@ -37,11 +37,25 @@ func TestNewWithHttpTimeout(t *testing.T) {
 func TestNewWithPwnedPadding(t *testing.T) {
 	hc := New(WithPwnedPadding())
 	if !hc.PwnedPassAPIOpts.WithPadding {
-		t.Errorf("hibp client pwned padding option was not set properly. Expected %v, got: %v",
+		t.Errorf("hibp client pwned padding option was not set properly. Expected %t, got: %t",
 			true, hc.PwnedPassAPIOpts.WithPadding)
 	}
 }
 
+// TestNewWithPwnedNTLMHash tests the New() function with the PwnedPadding option
+func TestNewWithPwnedNTLMHash(t *testing.T) {
+	hc := New(WithPwnedNTLMHash())
+	if hc.PwnedPassAPIOpts.HashMode != HashModeNTLM {
+		t.Errorf("hibp client NTLM hash mode option was not set properly. Expected %d, got: %d",
+			HashModeNTLM, hc.PwnedPassAPIOpts.HashMode)
+	}
+	hc = New()
+	if hc.PwnedPassAPIOpts.HashMode != HashModeSHA1 {
+		t.Errorf("hibp client SHA-1 hash mode option was not set properly. Expected %d, got: %d",
+			HashModeSHA1, hc.PwnedPassAPIOpts.HashMode)
+	}
+}
+
 // TestNewWithApiKey tests the New() function with the API key set
 func TestNewWithApiKey(t *testing.T) {
 	apiKey := os.Getenv("HIBP_API_KEY")
diff --git a/md4/LICENSE b/md4/LICENSE
new file mode 100644
index 0000000..6a66aea
--- /dev/null
+++ b/md4/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/md4/md4.go b/md4/md4.go
new file mode 100644
index 0000000..e1cc3d0
--- /dev/null
+++ b/md4/md4.go
@@ -0,0 +1,122 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package md4 implements the MD4 hash algorithm as defined in RFC 1320.
+//
+// NOTE: MD4 is cryptographically broken and should should only be used
+// where compatibility with legacy systems, not security, is the goal. Instead,
+// use a secure hash like SHA-256 (from crypto/sha256).
+package md4 // import "golang.org/x/crypto/md4"
+
+import (
+	"crypto"
+	"hash"
+)
+
+func init() {
+	crypto.RegisterHash(crypto.MD4, New)
+}
+
+// Size is the size of an MD4 checksum in bytes.
+const Size = 16
+
+// BlockSize is the blocksize of MD4 in bytes.
+const BlockSize = 64
+
+const (
+	_Chunk = 64
+	_Init0 = 0x67452301
+	_Init1 = 0xEFCDAB89
+	_Init2 = 0x98BADCFE
+	_Init3 = 0x10325476
+)
+
+// digest represents the partial evaluation of a checksum.
+type digest struct {
+	s   [4]uint32
+	x   [_Chunk]byte
+	nx  int
+	len uint64
+}
+
+func (d *digest) Reset() {
+	d.s[0] = _Init0
+	d.s[1] = _Init1
+	d.s[2] = _Init2
+	d.s[3] = _Init3
+	d.nx = 0
+	d.len = 0
+}
+
+// New returns a new hash.Hash computing the MD4 checksum.
+func New() hash.Hash {
+	d := new(digest)
+	d.Reset()
+	return d
+}
+
+func (d *digest) Size() int { return Size }
+
+func (d *digest) BlockSize() int { return BlockSize }
+
+func (d *digest) Write(p []byte) (nn int, err error) {
+	nn = len(p)
+	d.len += uint64(nn)
+	if d.nx > 0 {
+		n := len(p)
+		if n > _Chunk-d.nx {
+			n = _Chunk - d.nx
+		}
+		for i := 0; i < n; i++ {
+			d.x[d.nx+i] = p[i]
+		}
+		d.nx += n
+		if d.nx == _Chunk {
+			_Block(d, d.x[0:])
+			d.nx = 0
+		}
+		p = p[n:]
+	}
+	n := _Block(d, p)
+	p = p[n:]
+	if len(p) > 0 {
+		d.nx = copy(d.x[:], p)
+	}
+	return
+}
+
+func (d *digest) Sum(in []byte) []byte {
+	// Make a copy of d0, so that caller can keep writing and summing.
+	dc := new(digest)
+	*dc = *d
+
+	// Padding.  Add a 1 bit and 0 bits until 56 bytes mod 64.
+	plen := dc.len
+	var tmp [64]byte
+	tmp[0] = 0x80
+	if plen%64 < 56 {
+		_, _ = dc.Write(tmp[0 : 56-plen%64])
+	} else {
+		_, _ = dc.Write(tmp[0 : 64+56-plen%64])
+	}
+
+	// Length in bits.
+	plen <<= 3
+	for i := uint(0); i < 8; i++ {
+		tmp[i] = byte(plen >> (8 * i))
+	}
+	_, _ = dc.Write(tmp[0:8])
+
+	if dc.nx != 0 {
+		panic("dc.nx != 0")
+	}
+
+	for _, s := range dc.s {
+		in = append(in, byte(s>>0))
+		in = append(in, byte(s>>8))
+		in = append(in, byte(s>>16))
+		in = append(in, byte(s>>24))
+	}
+	return in
+}
diff --git a/md4/md4_test.go b/md4/md4_test.go
new file mode 100644
index 0000000..684dce5
--- /dev/null
+++ b/md4/md4_test.go
@@ -0,0 +1,71 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package md4
+
+import (
+	"fmt"
+	"io"
+	"testing"
+)
+
+type md4Test struct {
+	out string
+	in  string
+}
+
+var golden = []md4Test{
+	{"31d6cfe0d16ae931b73c59d7e0c089c0", ""},
+	{"bde52cb31de33e46245e05fbdbd6fb24", "a"},
+	{"ec388dd78999dfc7cf4632465693b6bf", "ab"},
+	{"a448017aaf21d8525fc10ae87aa6729d", "abc"},
+	{"41decd8f579255c5200f86a4bb3ba740", "abcd"},
+	{"9803f4a34e8eb14f96adba49064a0c41", "abcde"},
+	{"804e7f1c2586e50b49ac65db5b645131", "abcdef"},
+	{"752f4adfe53d1da0241b5bc216d098fc", "abcdefg"},
+	{"ad9daf8d49d81988590a6f0e745d15dd", "abcdefgh"},
+	{"1e4e28b05464316b56402b3815ed2dfd", "abcdefghi"},
+	{"dc959c6f5d6f9e04e4380777cc964b3d", "abcdefghij"},
+	{"1b5701e265778898ef7de5623bbe7cc0", "Discard medicine more than two years old."},
+	{"d7f087e090fe7ad4a01cb59dacc9a572", "He who has a shady past knows that nice guys finish last."},
+	{"a6f8fd6df617c72837592fc3570595c9", "I wouldn't marry him with a ten foot pole."},
+	{"c92a84a9526da8abc240c05d6b1a1ce0", "Free! Free!/A trip/to Mars/for 900/empty jars/Burma Shave"},
+	{"f6013160c4dcb00847069fee3bb09803", "The days of the digital watch are numbered.  -Tom Stoppard"},
+	{"2c3bb64f50b9107ed57640fe94bec09f", "Nepal premier won't resign."},
+	{"45b7d8a32c7806f2f7f897332774d6e4", "For every action there is an equal and opposite government program."},
+	{"b5b4f9026b175c62d7654bdc3a1cd438", "His money is twice tainted: 'taint yours and 'taint mine."},
+	{"caf44e80f2c20ce19b5ba1cab766e7bd", "There is no reason for any individual to have a computer in their home. -Ken Olsen, 1977"},
+	{"191fae6707f496aa54a6bce9f2ecf74d", "It's a tiny change to the code and not completely disgusting. - Bob Manchek"},
+	{"9ddc753e7a4ccee6081cd1b45b23a834", "size:  a.out:  bad magic"},
+	{"8d050f55b1cadb9323474564be08a521", "The major problem is with sendmail.  -Mark Horton"},
+	{"ad6e2587f74c3e3cc19146f6127fa2e3", "Give me a rock, paper and scissors and I will move the world.  CCFestoon"},
+	{"1d616d60a5fabe85589c3f1566ca7fca", "If the enemy is within range, then so are you."},
+	{"aec3326a4f496a2ced65a1963f84577f", "It's well we cannot hear the screams/That we create in others' dreams."},
+	{"77b4fd762d6b9245e61c50bf6ebf118b", "You remind me of a TV show, but that's all right: I watch it anyway."},
+	{"e8f48c726bae5e516f6ddb1a4fe62438", "C is as portable as Stonehedge!!"},
+	{"a3a84366e7219e887423b01f9be7166e", "Even if I could be Shakespeare, I think I should still choose to be Faraday. - A. Huxley"},
+	{"a6b7aa35157e984ef5d9b7f32e5fbb52", "The fugacity of a constituent in a mixture of gases at a given temperature is proportional to its mole fraction.  Lewis-Randall Rule"},
+	{"75661f0545955f8f9abeeb17845f3fd6", "How can you write a big system without C++?  -Paul Glick"},
+}
+
+func TestGolden(t *testing.T) {
+	for i := 0; i < len(golden); i++ {
+		g := golden[i]
+		c := New()
+		for j := 0; j < 3; j++ {
+			if j < 2 {
+				_, _ = io.WriteString(c, g.in)
+			} else {
+				_, _ = io.WriteString(c, g.in[0:len(g.in)/2])
+				c.Sum(nil)
+				_, _ = io.WriteString(c, g.in[len(g.in)/2:])
+			}
+			s := fmt.Sprintf("%x", c.Sum(nil))
+			if s != g.out {
+				t.Fatalf("md4[%d](%s) = %s want %s", j, g.in, s, g.out)
+			}
+			c.Reset()
+		}
+	}
+}
diff --git a/md4/md4block.go b/md4/md4block.go
new file mode 100644
index 0000000..547220f
--- /dev/null
+++ b/md4/md4block.go
@@ -0,0 +1,95 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// MD4 block step.
+// In its own file so that a faster assembly or C version
+// can be substituted easily.
+
+package md4
+
+import "math/bits"
+
+var (
+	shift1 = []int{3, 7, 11, 19}
+	shift2 = []int{3, 5, 9, 13}
+	shift3 = []int{3, 9, 11, 15}
+)
+
+var (
+	xIndex2 = []uint{0, 4, 8, 12, 1, 5, 9, 13, 2, 6, 10, 14, 3, 7, 11, 15}
+	xIndex3 = []uint{0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15}
+)
+
+func _Block(dig *digest, p []byte) int {
+	a := dig.s[0]
+	b := dig.s[1]
+	c := dig.s[2]
+	d := dig.s[3]
+	n := 0
+	var X [16]uint32
+	for len(p) >= _Chunk {
+		aa, bb, cc, dd := a, b, c, d
+
+		j := 0
+		for i := 0; i < 16; i++ {
+			X[i] = uint32(p[j]) | uint32(p[j+1])<<8 | uint32(p[j+2])<<16 | uint32(p[j+3])<<24
+			j += 4
+		}
+
+		// If this needs to be made faster in the future,
+		// the usual trick is to unroll each of these
+		// loops by a factor of 4; that lets you replace
+		// the shift[] lookups with constants and,
+		// with suitable variable renaming in each
+		// unrolled body, delete the a, b, c, d = d, a, b, c
+		// (or you can let the optimizer do the renaming).
+		//
+		// The index variables are uint so that % by a power
+		// of two can be optimized easily by a compiler.
+
+		// Round 1.
+		for i := uint(0); i < 16; i++ {
+			x := i
+			s := shift1[i%4]
+			f := ((c ^ d) & b) ^ d
+			a += f + X[x]
+			a = bits.RotateLeft32(a, s)
+			a, b, c, d = d, a, b, c
+		}
+
+		// Round 2.
+		for i := uint(0); i < 16; i++ {
+			x := xIndex2[i]
+			s := shift2[i%4]
+			g := (b & c) | (b & d) | (c & d)
+			a += g + X[x] + 0x5a827999
+			a = bits.RotateLeft32(a, s)
+			a, b, c, d = d, a, b, c
+		}
+
+		// Round 3.
+		for i := uint(0); i < 16; i++ {
+			x := xIndex3[i]
+			s := shift3[i%4]
+			h := b ^ c ^ d
+			a += h + X[x] + 0x6ed9eba1
+			a = bits.RotateLeft32(a, s)
+			a, b, c, d = d, a, b, c
+		}
+
+		a += aa
+		b += bb
+		c += cc
+		d += dd
+
+		p = p[_Chunk:]
+		n += _Chunk
+	}
+
+	dig.s[0] = a
+	dig.s[1] = b
+	dig.s[2] = c
+	dig.s[3] = d
+	return n
+}
diff --git a/password.go b/password.go
index cac5f8e..319a143 100644
--- a/password.go
+++ b/password.go
@@ -8,11 +8,17 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
+	"unicode/utf16"
+
+	"github.com/wneessen/go-hibp/md4"
 )
 
 // PwnedPassAPI is a HIBP Pwned Passwords API client
 type PwnedPassAPI struct {
-	hibp *Client // References back to the parent HIBP client
+	// References back to the parent HIBP client
+	hibp *Client
+	// Query parameter map for additional query parameters passed to request
+	ParamMap map[string]string
 }
 
 // Match represents a match in the Pwned Passwords API
@@ -21,17 +27,44 @@ type Match struct {
 	Count int64  // Represents the number of leaked accounts that hold/held this password
 }
 
+type HashMode int
+
+const (
+	// HashModeSHA1 is the default hash mode expecting SHA-1 hashes
+	HashModeSHA1 HashMode = iota
+	// HashModeNTLM represents the mode that expects and returns NTLM hashes
+	HashModeNTLM
+)
+
 // PwnedPasswordOptions is a struct of additional options for the PP API
 type PwnedPasswordOptions struct {
+	// HashMode controls whether the provided hash is in SHA-1 or NTLM format
+	// HashMode defaults to SHA-1 and can be overridden using the WithNTLMHash() Option
+	// See: https://haveibeenpwned.com/API/v3#PwnedPasswordsNTLM
+	HashMode HashMode
+
 	// WithPadding controls if the PwnedPassword API returns with padding or not
 	// See: https://haveibeenpwned.com/API/v3#PwnedPasswordsPadding
 	WithPadding bool
 }
 
 // CheckPassword checks the Pwned Passwords database against a given password string
+//
+// This method will automatically decide whether the hash is in SHA-1 or NTLM format based on
+// the Option when the Client was initialized
 func (p *PwnedPassAPI) CheckPassword(pw string) (*Match, *http.Response, error) {
-	shaSum := fmt.Sprintf("%x", sha1.Sum([]byte(pw)))
-	return p.CheckSHA1(shaSum)
+	switch p.hibp.PwnedPassAPIOpts.HashMode {
+	case HashModeSHA1:
+		shaSum := fmt.Sprintf("%x", sha1.Sum([]byte(pw)))
+		return p.CheckSHA1(shaSum)
+	case HashModeNTLM:
+		d := md4.New()
+		d.Write(stringToUTF16(pw))
+		md4Sum := fmt.Sprintf("%x", d.Sum(nil))
+		return p.CheckNTLM(md4Sum)
+	default:
+		return nil, nil, ErrUnsupportedHashMode
+	}
 }
 
 // CheckSHA1 checks the Pwned Passwords database against a given SHA1 checksum of a password string
@@ -40,13 +73,34 @@ func (p *PwnedPassAPI) CheckSHA1(h string) (*Match, *http.Response, error) {
 		return nil, nil, ErrSHA1LengthMismatch
 	}
 
+	p.hibp.PwnedPassAPIOpts.HashMode = HashModeSHA1
 	pwMatches, hr, err := p.ListHashesPrefix(h[:5])
 	if err != nil {
 		return &Match{}, hr, err
 	}
 
 	for _, m := range pwMatches {
-		if m.Hash == h {
+		if m.Hash == strings.ToLower(h) {
+			return &m, hr, nil
+		}
+	}
+	return nil, hr, nil
+}
+
+// CheckNTLM checks the Pwned Passwords database against a given NTLM hash of a password string
+func (p *PwnedPassAPI) CheckNTLM(h string) (*Match, *http.Response, error) {
+	if len(h) != 32 {
+		return nil, nil, ErrNTLMLengthMismatch
+	}
+
+	p.hibp.PwnedPassAPIOpts.HashMode = HashModeNTLM
+	pwMatches, hr, err := p.ListHashesPrefix(h[:5])
+	if err != nil {
+		return &Match{}, hr, err
+	}
+
+	for _, m := range pwMatches {
+		if m.Hash == strings.ToLower(h) {
 			return &m, hr, nil
 		}
 	}
@@ -56,11 +110,24 @@ func (p *PwnedPassAPI) CheckSHA1(h string) (*Match, *http.Response, error) {
 // ListHashesPassword checks the Pwned Password API endpoint for all hashes based on a given
 // password string and returns the a slice of Match as well as the http.Response
 //
+// This method will automatically decide whether the hash is in SHA-1 or NTLM format based on
+// the Option when the Client was initialized
+//
 // NOTE: If the `WithPwnedPadding` option is set to true, the returned list will be padded and might
 // contain junk data
 func (p *PwnedPassAPI) ListHashesPassword(pw string) ([]Match, *http.Response, error) {
-	shaSum := fmt.Sprintf("%x", sha1.Sum([]byte(pw)))
-	return p.ListHashesSHA1(shaSum)
+	switch p.hibp.PwnedPassAPIOpts.HashMode {
+	case HashModeSHA1:
+		shaSum := fmt.Sprintf("%x", sha1.Sum([]byte(pw)))
+		return p.ListHashesSHA1(shaSum)
+	case HashModeNTLM:
+		d := md4.New()
+		d.Write(stringToUTF16(pw))
+		md4Sum := fmt.Sprintf("%x", d.Sum(nil))
+		return p.ListHashesNTLM(md4Sum)
+	default:
+		return nil, nil, ErrUnsupportedHashMode
+	}
 }
 
 // ListHashesSHA1 checks the Pwned Password API endpoint for all hashes based on a given
@@ -72,6 +139,7 @@ func (p *PwnedPassAPI) ListHashesSHA1(h string) ([]Match, *http.Response, error)
 	if len(h) != 40 {
 		return nil, nil, ErrSHA1LengthMismatch
 	}
+	p.hibp.PwnedPassAPIOpts.HashMode = HashModeSHA1
 	dst := make([]byte, hex.DecodedLen(len(h)))
 	if _, err := hex.Decode(dst, []byte(h)); err != nil {
 		return nil, nil, ErrSHA1Invalid
@@ -79,8 +147,28 @@ func (p *PwnedPassAPI) ListHashesSHA1(h string) ([]Match, *http.Response, error)
 	return p.ListHashesPrefix(h[:5])
 }
 
+// ListHashesNTLM checks the Pwned Password API endpoint for all hashes based on a given
+// NTLM hash and returns the a slice of Match as well as the http.Response
+//
+// NOTE: If the `WithPwnedPadding` option is set to true, the returned list will be padded and might
+// contain junk data
+func (p *PwnedPassAPI) ListHashesNTLM(h string) ([]Match, *http.Response, error) {
+	if len(h) != 32 {
+		return nil, nil, ErrNTLMLengthMismatch
+	}
+	p.hibp.PwnedPassAPIOpts.HashMode = HashModeNTLM
+	dst := make([]byte, hex.DecodedLen(len(h)))
+	if _, err := hex.Decode(dst, []byte(h)); err != nil {
+		return nil, nil, ErrNTLMInvalid
+	}
+	return p.ListHashesPrefix(h[:5])
+}
+
 // ListHashesPrefix checks the Pwned Password API endpoint for all hashes based on a given
-// SHA1 checksum prefix and returns the a slice of Match as well as the http.Response
+// SHA-1 or NTLM hash prefix and returns the a slice of Match as well as the http.Response
+//
+// To decide which HashType is queried for, make sure to set the appropriate HashMode in
+// the PwnedPassAPI struct
 //
 // NOTE: If the `WithPwnedPadding` option is set to true, the returned list will be padded and might
 // contain junk data
@@ -89,8 +177,16 @@ func (p *PwnedPassAPI) ListHashesPrefix(pf string) ([]Match, *http.Response, err
 		return nil, nil, ErrPrefixLengthMismatch
 	}
 
+	switch p.hibp.PwnedPassAPIOpts.HashMode {
+	case HashModeSHA1:
+		delete(p.ParamMap, "mode")
+	case HashModeNTLM:
+		p.ParamMap["mode"] = "ntlm"
+	default:
+		delete(p.ParamMap, "mode")
+	}
 	au := fmt.Sprintf("%s/range/%s", PasswdBaseURL, pf)
-	hreq, err := p.hibp.HTTPReq(http.MethodGet, au, nil)
+	hreq, err := p.hibp.HTTPReq(http.MethodGet, au, p.ParamMap)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -129,3 +225,14 @@ func (p *PwnedPassAPI) ListHashesPrefix(pf string) ([]Match, *http.Response, err
 
 	return pm, hr, nil
 }
+
+// stringToUTF16 converts a given string to a UTF-16 little-endian encoded byte slice
+func stringToUTF16(s string) []byte {
+	e := utf16.Encode([]rune(s))
+	r := make([]byte, len(e)*2)
+	for i := 0; i < len(e); i++ {
+		r[i*2] = byte(e[i])
+		r[i*2+1] = byte(e[i] << 8)
+	}
+	return r
+}
diff --git a/password_test.go b/password_test.go
index b67d3f6..e52e736 100644
--- a/password_test.go
+++ b/password_test.go
@@ -17,9 +17,17 @@ const (
 	// Represents the string: test
 	PwHashInsecure = "a94a8fe5ccb19ba61c4c0873d391e987982fbbd3"
 
+	// PwHashInsecure is the NTLM hash of an insecure password
+	// Represents the string: test
+	PwHashInsecureNTLM = "0cb6948805f797bf2a82807973b89537"
+
 	// PwHashSecure is the SHA1 checksum of a secure password
 	// Represents the string: F/0Ws#.%{Z/NVax=OU8Ajf1qTRLNS12p/?s/adX
 	PwHashSecure = "90efc095c82eab44e882fda507cfab1a2cd31fc0"
+
+	// PwHashSecureNTLM is the NTLM hash of a secure password
+	// Represents the string: F/0Ws#.%{Z/NVax=OU8Ajf1qTRLNS12p/?s/adX
+	PwHashSecureNTLM = "997f11041d9aa830842e682d1b4207df"
 )
 
 // TestPwnedPassAPI_CheckPassword verifies the Pwned Passwords API with the CheckPassword method
@@ -29,7 +37,7 @@ func TestPwnedPassAPI_CheckPassword(t *testing.T) {
 		pwString string
 		isLeaked bool
 	}{
-		{"weak password 'test123' is expected to be leaked", PwStringInsecure, true},
+		{"weak password 'test' is expected to be leaked", PwStringInsecure, true},
 		{
 			"strong, unknown password is expected to be not leaked",
 			PwStringSecure, false,
@@ -53,6 +61,38 @@ func TestPwnedPassAPI_CheckPassword(t *testing.T) {
 	}
 }
 
+// TestPwnedPassAPI_CheckPassword_NTLM verifies the Pwned Passwords API with the CheckPassword method
+// with NTLM hashes enabled
+func TestPwnedPassAPI_CheckPassword_NTLM(t *testing.T) {
+	testTable := []struct {
+		testName string
+		pwString string
+		isLeaked bool
+	}{
+		{"weak password 'test' is expected to be leaked", PwStringInsecure, true},
+		{
+			"strong, unknown password is expected to be not leaked",
+			PwStringSecure, false,
+		},
+	}
+	hc := New(WithPwnedNTLMHash())
+	for _, tc := range testTable {
+		t.Run(tc.testName, func(t *testing.T) {
+			m, _, err := hc.PwnedPassAPI.CheckPassword(tc.pwString)
+			if err != nil {
+				t.Error(err)
+			}
+			if m == nil && tc.isLeaked {
+				t.Errorf("password is expected to be leaked but 0 leaks were returned in Pwned Passwords DB")
+			}
+			if m != nil && m.Count > 0 && !tc.isLeaked {
+				t.Errorf("password is not expected to be leaked but %d leaks were found in Pwned Passwords DB",
+					m.Count)
+			}
+		})
+	}
+}
+
 // TestPwnedPassAPI_CheckSHA1 verifies the Pwned Passwords API with the CheckSHA1 method
 func TestPwnedPassAPI_CheckSHA1(t *testing.T) {
 	testTable := []struct {
@@ -89,6 +129,52 @@ func TestPwnedPassAPI_CheckSHA1(t *testing.T) {
 				t.Errorf("password is not expected to be leaked but %d leaks were found in Pwned Passwords DB",
 					m.Count)
 			}
+			if m != nil && m.Hash != tc.pwHash {
+				t.Errorf("password hashes don't match, expected: %s, got %s", tc.pwHash, m.Hash)
+			}
+		})
+	}
+}
+
+// TestPwnedPassAPI_CheckNTLM verifies the Pwned Passwords API with the CheckNTLM method
+func TestPwnedPassAPI_CheckNTLM(t *testing.T) {
+	testTable := []struct {
+		testName   string
+		pwHash     string
+		isLeaked   bool
+		shouldFail bool
+	}{
+		{
+			"weak password 'test' is expected to be leaked",
+			PwHashInsecureNTLM, true, false,
+		},
+		{
+			"strong, unknown password is expected to be not leaked",
+			PwHashSecureNTLM, false, false,
+		},
+		{
+			"empty string should fail",
+			"", false, true,
+		},
+	}
+	hc := New()
+	for _, tc := range testTable {
+		t.Run(tc.testName, func(t *testing.T) {
+			m, _, err := hc.PwnedPassAPI.CheckNTLM(tc.pwHash)
+			if err != nil && !tc.shouldFail {
+				t.Error(err)
+				return
+			}
+			if m == nil && tc.isLeaked {
+				t.Errorf("password is expected to be leaked but 0 leaks were returned in Pwned Passwords DB")
+			}
+			if m != nil && m.Count > 0 && !tc.isLeaked {
+				t.Errorf("password is not expected to be leaked but %d leaks were found in Pwned Passwords DB",
+					m.Count)
+			}
+			if m != nil && m.Hash != tc.pwHash {
+				t.Errorf("password hashes don't match, expected: %s, got %s", tc.pwHash, m.Hash)
+			}
 		})
 	}
 }
@@ -188,6 +274,44 @@ func TestPwnedPassAPI_ListHashesSHA1_Errors(t *testing.T) {
 	})
 }
 
+// TestPwnedPassAPI_ListHashesNTLM_Errors tests the ListHashesNTLM method's errors
+func TestPwnedPassAPI_ListHashesNTLM_Errors(t *testing.T) {
+	hc := New()
+
+	// Empty hash
+	t.Run("empty hash", func(t *testing.T) {
+		_, _, err := hc.PwnedPassAPI.ListHashesNTLM("")
+		if err == nil {
+			t.Errorf("ListHashesNTLM with empty hash should fail but didn't")
+		}
+		if !errors.Is(err, ErrNTLMLengthMismatch) {
+			t.Errorf("ListHashesNTLM with empty hash should return ErrNTLMLengthMismatch error but didn't")
+		}
+	})
+
+	// Too long hash
+	t.Run("too long hash", func(t *testing.T) {
+		_, _, err := hc.PwnedPassAPI.ListHashesNTLM("FF36DC7D3284A39991ADA90CAF20D1E3C0DADEFAB")
+		if err == nil {
+			t.Errorf("ListHashesNTLM with too long hash should fail but didn't")
+		}
+		if !errors.Is(err, ErrNTLMLengthMismatch) {
+			t.Errorf("ListHashesNTLM with too long hash should return ErrNTLMLengthMismatch error but didn't")
+		}
+	})
+
+	// Invalid hash
+	t.Run("invalid hash", func(t *testing.T) {
+		_, _, err := hc.PwnedPassAPI.ListHashesNTLM("3284A39991ADA90CAF20D1E3C0DADEFZ")
+		if err == nil {
+			t.Errorf("ListHashesNTLM with invalid hash should fail but didn't")
+		}
+		if !errors.Is(err, ErrNTLMInvalid) {
+			t.Errorf("ListHashesNTLM with invalid hash should return ErrSHA1Invalid error but didn't")
+		}
+	})
+}
+
 // TestPwnedPassApi_ListHashesSHA1 tests the PwnedPassAPI.ListHashesSHA1 metethod
 func TestPwnedPassAPI_ListHashesSHA1(t *testing.T) {
 	hc := New()
@@ -208,6 +332,26 @@ func TestPwnedPassAPI_ListHashesSHA1(t *testing.T) {
 	}
 }
 
+// TestPwnedPassApi_ListHashesNTLM tests the PwnedPassAPI.ListHashesNTLM metethod
+func TestPwnedPassAPI_ListHashesNTLM(t *testing.T) {
+	hc := New(WithPwnedNTLMHash())
+
+	// List length should be >0
+	l, _, err := hc.PwnedPassAPI.ListHashesNTLM(PwHashInsecureNTLM)
+	if err != nil {
+		t.Errorf("ListHashesNTLM was not supposed to fail, but did: %s", err)
+	}
+	if len(l) <= 0 {
+		t.Errorf("ListHashesNTLM was supposed to return a list longer than 0")
+	}
+
+	// Hash has wrong size
+	_, _, err = hc.PwnedPassAPI.ListHashesNTLM(PwStringInsecure)
+	if err == nil {
+		t.Errorf("ListHashesNTLM was supposed to fail, but didn't")
+	}
+}
+
 // TestPwnedPassAPI_ListHashesPassword tests the PwnedPassAPI.ListHashesPassword metethod
 func TestPwnedPassAPI_ListHashesPassword(t *testing.T) {
 	hc := New()
@@ -273,3 +417,19 @@ func ExamplePwnedPassAPI_checkSHA1() {
 		// Output: Your password with the hash "a94a8fe5ccb19ba61c4c0873d391e987982fbbd3" was found 86495 times in the pwned passwords DB
 	}
 }
+
+// ExamplePwnedPassAPI_checkNTLM is a code example to show how to check a given password NTLM hash
+// against the HIBP passwords API using the CheckNTLM() method
+func ExamplePwnedPassAPI_checkNTLM() {
+	hc := New()
+	pwHash := "0cb6948805f797bf2a82807973b89537" // represents the PW: "test"
+	m, _, err := hc.PwnedPassAPI.CheckNTLM(pwHash)
+	if err != nil {
+		panic(err)
+	}
+	if m != nil && m.Count != 0 {
+		fmt.Printf("Your password with the hash %q was found %d times in the pwned passwords DB\n",
+			m.Hash, m.Count)
+		// Output: Your password with the hash "0cb6948805f797bf2a82807973b89537" was found 86495 times in the pwned passwords DB
+	}
+}