diff --git a/src/ImageHash.php b/src/ImageHash.php
index 55ea5f8..1bed620 100644
--- a/src/ImageHash.php
+++ b/src/ImageHash.php
@@ -113,8 +113,8 @@ public function distance($hash1, $hash2)
             }
         } else {
             if ($this->mode === self::HEXADECIMAL) {
-                $hash1 = hexdec($hash1);
-                $hash2 = hexdec($hash2);
+                $hash1 = $this->hexdec($hash1);
+                $hash2 = $this->hexdec($hash2);
             }
 
             $dh = 0;
@@ -129,6 +129,22 @@ public function distance($hash1, $hash2)
         return $dh;
     }
 
+    /**
+     * Convert hexadecimal to signed decimal.
+     *
+     * @param string $hex
+     * @return int
+     */
+    public function hexdec($hex)
+    {
+        if (strlen($hex) == 16 && hexdec($hex[0]) > 8) {
+            list($higher, $lower) = array_values(unpack('N2', hex2bin($hex)));
+            return $higher << 32 | $lower;
+        }
+
+        return hexdec($hex);
+    }
+
     /**
      * Get a GD2 resource from file.
      *
diff --git a/tests/ImageHashTest.php b/tests/ImageHashTest.php
index 140eee6..784f8ae 100644
--- a/tests/ImageHashTest.php
+++ b/tests/ImageHashTest.php
@@ -31,4 +31,26 @@ public function testHashStringSameAsFile()
 
         $this->assertSame($this->imageHash->hash($path), $this->imageHash->hashFromString(file_get_contents($path)));
     }
+
+    public function testHexdecForNegativeIntegers()
+    {
+        // native hexdec dechex conversion working for positive integers
+        $this->assertEquals(1, hexdec(dechex(1)));
+        // but not working for negative
+        $this->assertNotEquals(-1, hexdec(dechex(-1)));
+
+        // custom hexdec implementation works for both
+        $this->assertEquals(1, $this->imageHash->hexdec(dechex(1)));
+        $this->assertEquals(-1, $this->imageHash->hexdec(dechex(-1)));
+    }
+
+    public function testDistanceOfNegativeHashes()
+    {
+        $imageHash = new ImageHash(null, ImageHash::HEXADECIMAL);
+        $hash1 = 'ffffffffffffffff'; // -1
+        $hash2 = 'fffffffffffffff0'; // -16
+
+        $distance = $imageHash->distance($hash1, $hash2);
+        $this->assertEquals(4, $distance);
+    }
 }