diff --git a/Lib/test/test_StringIO.py b/Lib/test/test_StringIO.py
index c318eaafc79..6842c5ef88d 100644
--- a/Lib/test/test_StringIO.py
+++ b/Lib/test/test_StringIO.py
@@ -49,9 +49,10 @@ class TestGenericStringIO(unittest.TestCase):
         f.seek(10)
         f.truncate()
         eq(f.getvalue(), 'abcdefghij')
-        f.seek(0)
         f.truncate(5)
         eq(f.getvalue(), 'abcde')
+        f.write('xyz')
+        eq(f.getvalue(), 'abcdexyz')
         f.close()
         self.assertRaises(ValueError, f.write, 'frobnitz')
 
diff --git a/Misc/NEWS b/Misc/NEWS
index a8aee9c3209..6b1dcff26e1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -41,6 +41,11 @@ Core and builtins
 Extension modules
 -----------------
 
+- Patch 1012740:  truncate() on a writeable cStringIO now resets the
+  position to the end of the stream.  This is consistent with the original
+  StringIO module and avoids inadvertently resurrecting data that was
+  supposed to have been truncated away.
+
 - Added socket.socketpair().
 
 Library
@@ -59,7 +64,7 @@ Library
 - A new function tkFont.nametofont was added to return an existing
   font. The Font class constructor now has an additional exists argument
   which, if True, requests to return/configure an existing font, rather
-  than creating a new one. 
+  than creating a new one.
 
 - Updated the decimal package's min() and max() methods to match the
   latest revision of the General Decimal Arithmetic Specification.
diff --git a/Modules/cStringIO.c b/Modules/cStringIO.c
index 7e75879215a..b7333fdd380 100644
--- a/Modules/cStringIO.c
+++ b/Modules/cStringIO.c
@@ -289,6 +289,7 @@ IO_truncate(IOobject *self, PyObject *args) {
         if (pos < 0) pos = self->pos;
 
         if (self->string_size > pos) self->string_size = pos;
+        self->pos = self->string_size;
 
         Py_INCREF(Py_None);
         return Py_None;