diff -r 1d811e1097ed Lib/test/test_memoryio.py
--- a/Lib/test/test_memoryio.py	Sun Jul 29 19:04:57 2012 +0200
+++ b/Lib/test/test_memoryio.py	Sun Jul 29 22:34:41 2012 +0300
@@ -701,6 +701,19 @@
         memio.close()
         self.assertRaises(ValueError, memio.__setstate__, ("closed", "", 0, None))
 
+    check_sizeof = support.check_sizeof
+
+    @support.cpython_only
+    def test_sizeof(self):
+        basesize = support.calcobjsize('P2nNi 2P 4c5P')
+        check = self.check_sizeof
+        self.assertEqual(object.__sizeof__(io.StringIO()), basesize)
+        check(io.StringIO(), basesize + 2 * 4 )
+        check(io.StringIO('a' * 1000), basesize + (1000 + 2) * 4)
+        check(io.StringIO('\x80' * 1000), basesize + (1000 + 2) * 4)
+        check(io.StringIO('\u0100' * 1000), basesize + (1000 + 2) * 4)
+        check(io.StringIO('\U00010000' * 1000), basesize + (1000 + 2) * 4)
+
 
 class CStringIOPickleTest(PyStringIOPickleTest):
     UnsupportedOperation = io.UnsupportedOperation
diff -r 1d811e1097ed Modules/_io/stringio.c
--- a/Modules/_io/stringio.c	Sun Jul 29 19:04:57 2012 +0200
+++ b/Modules/_io/stringio.c	Sun Jul 29 22:34:41 2012 +0300
@@ -759,6 +759,17 @@
     return 0;
 }
 
+static PyObject *
+stringio_sizeof(stringio *self, void *unused)
+{
+    Py_ssize_t res;
+
+    res = sizeof(stringio);
+    if (self->buf)
+        res += self->buf_size * sizeof(Py_UCS4);
+    return PyLong_FromSsize_t(res);
+}
+
 /* Properties and pseudo-properties */
 static PyObject *
 stringio_seekable(stringio *self, PyObject *args)
@@ -962,6 +973,7 @@
 
     {"__getstate__", (PyCFunction)stringio_getstate, METH_NOARGS},
     {"__setstate__", (PyCFunction)stringio_setstate, METH_O},
+    {"__sizeof__", (PyCFunction)stringio_sizeof, METH_NOARGS},
     {NULL, NULL}        /* sentinel */
 };