diff -r 5206b9dbf1ac Lib/test/test_mmap.py
--- a/Lib/test/test_mmap.py	Thu Aug 16 07:27:47 2012 +0300
+++ b/Lib/test/test_mmap.py	Thu Aug 16 17:50:02 2012 +0300
@@ -1,5 +1,5 @@
 from test.support import (TESTFN, run_unittest, import_module, unlink,
-                          requires, _2G, _4G)
+                          requires, _2G, _4G, cpython_only)
 import unittest
 import os
 import re
@@ -633,6 +633,14 @@
             m2.close()
             m1.close()
 
+        @cpython_only
+        def test_sizeof(self):
+            m1 = mmap.mmap(-1, 100)
+            tagname = "foo"
+            m2 = mmap.mmap(-1, 100, tagname=tagname)
+            self.assertEqual(sys.getsize(m2),
+                             sys.getsize(m1) + len(tagname) + 1)
+
         def test_crasher_on_windows(self):
             # Should not crash (Issue 1733986)
             m = mmap.mmap(-1, 1000, tagname="foo")
diff -r 5206b9dbf1ac Modules/mmapmodule.c
--- a/Modules/mmapmodule.c	Thu Aug 16 07:27:47 2012 +0300
+++ b/Modules/mmapmodule.c	Thu Aug 16 17:50:02 2012 +0300
@@ -705,6 +705,19 @@
     return _PyObject_CallMethodId(self, &PyId_close, NULL);
 }
 
+#ifdef MS_WINDOWS
+static PyObject *
+mmap__sizeof__method(mmap_object *self, void *unused)
+{
+    Py_ssize_t res;
+
+    res = sizeof(mmap_object);
+    if (self->tagname)
+        res += strlen(self->tagname) + 1;
+    return PyLong_FromSsize_t(res);
+}
+#endif
+
 static struct PyMethodDef mmap_object_methods[] = {
     {"close",           (PyCFunction) mmap_close_method,        METH_NOARGS},
     {"find",            (PyCFunction) mmap_find_method,         METH_VARARGS},
@@ -722,6 +735,9 @@
     {"write_byte",      (PyCFunction) mmap_write_byte_method,   METH_VARARGS},
     {"__enter__",       (PyCFunction) mmap__enter__method,      METH_NOARGS},
     {"__exit__",        (PyCFunction) mmap__exit__method,       METH_VARARGS},
+#ifdef MS_WINDOWS
+    {"__sizeof__",      (PyCFunction) mmap__sizeof__method,     METH_NOARGS},
+#endif
     {NULL,         NULL}       /* sentinel */
 };