diff --git a/Doc/library/zlib.rst b/Doc/library/zlib.rst
index dda0b9c..e55e52a 100644
--- a/Doc/library/zlib.rst
+++ b/Doc/library/zlib.rst
@@ -42,6 +42,8 @@
    the algorithm is designed for use as a checksum algorithm, it is not suitable
    for use as a general hash algorithm.
 
+   Always returns an unsigned 32-bit integer.
+
 
 .. function:: compress(string[, level])
 
@@ -74,6 +76,8 @@
    the algorithm is designed for use as a checksum algorithm, it is not suitable
    for use as a general hash algorithm.
 
+   Always returns an unsigned 32-bit integer.
+
 
 .. function:: decompress(string[, wbits[, bufsize]])
 
diff --git a/Lib/test/test_zlib.py b/Lib/test/test_zlib.py
index 13656d6..6b7d4a6 100644
--- a/Lib/test/test_zlib.py
+++ b/Lib/test/test_zlib.py
@@ -38,6 +38,14 @@
         self.assertEqual(zlib.crc32(b"penguin"), zlib.crc32(b"penguin", 0))
         self.assertEqual(zlib.adler32(b"penguin"),zlib.adler32(b"penguin",1))
 
+    def test_crc32_adler32_unsigned(self):
+        foo = 'abcdefghijklmnop'
+        # explicitly test signed behavior
+        self.assertEqual(zlib.crc32(foo), 2486878355)
+        self.assertEqual(zlib.crc32('spam'), 1138425661)
+        self.assertEqual(zlib.adler32(foo+foo), 3573550353)
+        self.assertEqual(zlib.adler32('spam'), 72286642)
+
 
 
 class ExceptionTestCase(unittest.TestCase):
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index 441d61d..a978370 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -922,7 +922,7 @@
     if (!PyArg_ParseTuple(args, "s#|k:adler32", &buf, &len, &adler32val))
 	return NULL;
     adler32val = adler32(adler32val, buf, len);
-    return PyLong_FromLong(adler32val);
+    return PyLong_FromUnsignedLong(adler32val & 0xffffffff);
 }
 
 PyDoc_STRVAR(crc32__doc__,
@@ -940,7 +940,7 @@
     if (!PyArg_ParseTuple(args, "s#|k:crc32", &buf, &len, &crc32val))
 	return NULL;
     crc32val = crc32(crc32val, buf, len);
-    return PyLong_FromLong(crc32val);
+    return PyLong_FromUnsignedLong(crc32val & 0xffffffff);
 }