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.assertRaises(RuntimeError):
+            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)
@@ -2053,12 +2053,9 @@
     return (PyObject *)result;
 }
 
-/*
- * This is a *class method*.
- * Convert a parameter into something that ConvParam can handle.
- */
-static PyObject *
-PyCSimpleType_from_param(PyObject *type, PyObject *value)
+
+static PyObject*
+internal_csimpletype_from_param(PyObject *type, PyObject *value, PyObject *seen)
 {
     StgDictObject *dict;
     char *fmt;
@@ -2097,7 +2094,17 @@
 
     as_parameter = PyObject_GetAttrString(value, "_as_parameter_");
     if (as_parameter) {
-        value = PyCSimpleType_from_param(type, as_parameter);
+        if (PySet_Contains(seen, as_parameter)) {
+            PyErr_Format(PyExc_RuntimeError,
+                "_as_parameter_ contains recursive data"
+            );
+            return NULL;
+        }
+
+        if (PySet_Add(seen, as_parameter)) {
+            return NULL;
+        }
+        value = internal_csimpletype_from_param(type, as_parameter, seen);
         Py_DECREF(as_parameter);
         return value;
     }
@@ -2105,6 +2112,18 @@
                     "wrong type");
     return NULL;
 }
+/*
+ * This is a *class method*.
+ * Convert a parameter into something that ConvParam can handle.
+ */
+static PyObject *
+PyCSimpleType_from_param(PyObject *type, PyObject *value)
+{
+    PyObject *seen = PySet_New(NULL);
+    PyObject *result = internal_csimpletype_from_param(type, value, seen);
+    Py_DECREF(seen);
+    return result;
+}
 
 static PyMethodDef PyCSimpleType_methods[] = {
     { "from_param", PyCSimpleType_from_param, METH_O, from_param_doc },