Index: Lib/ctypes/test/test_as_parameter.py
===================================================================
--- Lib/ctypes/test/test_as_parameter.py	(revision 84911)
+++ Lib/ctypes/test/test_as_parameter.py	(working copy)
@@ -187,6 +187,18 @@
         self.assertEqual((s8i.a, s8i.b, s8i.c, s8i.d, s8i.e, s8i.f, s8i.g, s8i.h),
                              (9*2, 8*3, 7*4, 6*5, 5*6, 4*7, 3*8, 2*9))
 
+    def test_recursive_as_param(self):
+        from ctypes import c_int
+
+        class A(object):
+            pass
+
+        a = A()
+        a._as_parameter_ = a
+        with self.assertRaisesRegexp(RuntimeError, "while processing _as_parameter_"):
+            c_int.from_param(a)
+
+
 #~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 class AsParamWrapper(object):
Index: Modules/_ctypes/_ctypes.c
===================================================================
--- Modules/_ctypes/_ctypes.c	(revision 84911)
+++ Modules/_ctypes/_ctypes.c	(working copy)
@@ -2097,7 +2097,11 @@
 
     as_parameter = PyObject_GetAttrString(value, "_as_parameter_");
     if (as_parameter) {
+        if (Py_EnterRecursiveCall(" while processing _as_parameter_")) {
+            return NULL;
+        }
         value = PyCSimpleType_from_param(type, as_parameter);
+        Py_LeaveRecursiveCall();
         Py_DECREF(as_parameter);
         return value;
     }