diff -r 3bc1784be9aa Lib/test/test_zlib.py
--- a/Lib/test/test_zlib.py	Sun Oct 28 15:22:08 2012 +0100
+++ b/Lib/test/test_zlib.py	Mon Oct 29 13:57:55 2012 +0200
@@ -487,6 +487,18 @@
         dco.flush()
         self.assertFalse(dco.eof)
 
+    def test_decompress_unused_data(self):
+        x = zlib.compress(HAMLET_SCENE) + HAMLET_SCENE
+        for step in 1, 2, 100:
+            dco = zlib.decompressobj()
+            data = b''.join(dco.decompress(x[i : i + step])
+                            for i in range(0, len(x), step))
+            data += dco.flush()
+
+            self.assertTrue(dco.eof)
+            self.assertEqual(data, HAMLET_SCENE)
+            self.assertEqual(dco.unused_data, HAMLET_SCENE)
+
     if hasattr(zlib.compressobj(), "copy"):
         def test_compresscopy(self):
             # Test copying a compression object
diff -r 3bc1784be9aa Modules/zlibmodule.c
--- a/Modules/zlibmodule.c	Sun Oct 28 15:22:08 2012 +0100
+++ b/Modules/zlibmodule.c	Mon Oct 29 13:57:55 2012 +0200
@@ -693,12 +693,28 @@
        preserved.
     */
     if (err == Z_STREAM_END) {
-        Py_XDECREF(self->unused_data);  /* Free original empty string */
-        self->unused_data = PyBytes_FromStringAndSize(
-            (char *)self->zst.next_in, self->zst.avail_in);
-        if (self->unused_data == NULL) {
-            Py_DECREF(RetVal);
-            goto error;
+        if (self->zst.avail_in) {
+            size_t old_size = PyBytes_GET_SIZE(self->unused_data);
+            PyObject *new_data;
+            if(self->zst.avail_in > PY_SSIZE_T_MAX - old_size) {
+                PyErr_NoMemory();
+                Py_DECREF(RetVal);
+                RetVal = NULL;
+                goto error;
+            }
+            new_data = PyBytes_FromStringAndSize(NULL,
+                old_size + self->zst.avail_in);
+            if (new_data == NULL) {
+                Py_DECREF(RetVal);
+                RetVal = NULL;
+                goto error;
+            }
+            Py_MEMCPY(PyBytes_AS_STRING(new_data),
+                PyBytes_AS_STRING(self->unused_data), old_size);
+            Py_MEMCPY(PyBytes_AS_STRING(new_data) + old_size,
+                self->zst.next_in, self->zst.avail_in);
+            Py_DECREF(self->unused_data);
+            self->unused_data = new_data;
         }
         self->eof = 1;
         /* We will only get Z_BUF_ERROR if the output buffer was full