diff -r fa95b04e67fd Lib/test/test_memoryio.py
--- a/Lib/test/test_memoryio.py	Sun Jul 29 16:30:50 2012 +0200
+++ b/Lib/test/test_memoryio.py	Sun Jul 29 22:28:48 2012 +0300
@@ -638,6 +638,17 @@
         memio.close()
         self.assertRaises(ValueError, memio.__setstate__, (b"closed", 0, None))
 
+    check_sizeof = support.check_sizeof
+
+    @support.cpython_only
+    def test_sizeof(self):
+        basesize = support.calcobjsize(b'P2PP2P')
+        check = self.check_sizeof
+        self.assertEqual(object.__sizeof__(io.BytesIO()), basesize)
+        check(io.BytesIO(), basesize )
+        check(io.BytesIO(b'a'), basesize + 1 + 1 )
+        check(io.BytesIO(b'a' * 1000), basesize + 1000 + 1 )
+
 
 class CStringIOTest(PyStringIOTest):
     ioclass = io.StringIO
diff -r fa95b04e67fd Modules/_io/bytesio.c
--- a/Modules/_io/bytesio.c	Sun Jul 29 16:30:50 2012 +0200
+++ b/Modules/_io/bytesio.c	Sun Jul 29 22:28:48 2012 +0300
@@ -794,6 +794,17 @@
     return 0;
 }
 
+static PyObject *
+bytesio_sizeof(bytesio *self, void *unused)
+{
+    Py_ssize_t res;
+
+    res = sizeof(bytesio);
+    if (self->buf)
+        res += self->buf_size;
+    return PyLong_FromSsize_t(res);
+}
+
 static int
 bytesio_traverse(bytesio *self, visitproc visit, void *arg)
 {
@@ -835,6 +846,7 @@
     {"truncate",   (PyCFunction)bytesio_truncate,   METH_VARARGS, truncate_doc},
     {"__getstate__",  (PyCFunction)bytesio_getstate,  METH_NOARGS, NULL},
     {"__setstate__",  (PyCFunction)bytesio_setstate,  METH_O, NULL},
+    {"__sizeof__", (PyCFunction)bytesio_sizeof,     METH_NOARGS, NULL},
     {NULL, NULL}        /* sentinel */
 };