diff -r 66ca2fe3058b Modules/zlibmodule.c
--- a/Modules/zlibmodule.c	Mon Nov 01 06:10:44 2010 +0100
+++ b/Modules/zlibmodule.c	Mon Nov 01 10:56:53 2010 +0200
@@ -931,7 +931,17 @@
     /* Releasing the GIL for very small buffers is inefficient
        and may lower performance */
     if (pbuf.len > 1024*5) {
+        void *buf = pbuf.buf;
+        Py_ssize_t len = pbuf.len;
+
         Py_BEGIN_ALLOW_THREADS
+        /* Avoid truncation of length for very large buffers. adler32() takes
+           length as an unsigned int, which may be narrower than Py_ssize_t. */
+        while (len > (Py_ssize_t)UINT_MAX) {
+            adler32val = adler32(adler32val, buf, UINT_MAX);
+            buf += UINT_MAX;
+            len -= UINT_MAX;
+        }
         adler32val = adler32(adler32val, pbuf.buf, pbuf.len);
         Py_END_ALLOW_THREADS
     } else {
@@ -959,8 +969,18 @@
     /* Releasing the GIL for very small buffers is inefficient
        and may lower performance */
     if (pbuf.len > 1024*5) {
+        void *buf = pbuf.buf;
+        Py_ssize_t len = pbuf.len;
+
         Py_BEGIN_ALLOW_THREADS
-        signed_val = crc32(crc32val, pbuf.buf, pbuf.len);
+        /* Avoid truncation of length for very large buffers. crc32() takes
+           length as an unsigned int, which may be narrower than Py_ssize_t. */
+        while (len > (Py_ssize_t)UINT_MAX) {
+            crc32val = crc32(crc32val, buf, UINT_MAX);
+            buf += UINT_MAX;
+            len -= UINT_MAX;
+        }
+        signed_val = crc32(crc32val, buf, len);
         Py_END_ALLOW_THREADS
     } else {
         signed_val = crc32(crc32val, pbuf.buf, pbuf.len);