diff -r b0cc8c9ab912 Lib/abc.py
--- a/Lib/abc.py	Tue Aug 13 19:51:29 2013 -0400
+++ b/Lib/abc.py	Wed Aug 14 15:12:54 2013 +0200
@@ -166,6 +166,26 @@
         ABCMeta._abc_invalidation_counter += 1  # Invalidate negative cache
         return subclass
 
+    def get_virtual_subclasses(cls, recursive=False):
+        """Get registered virtual subclasses of an ABC."""
+        if not recursive:
+            return set(cls._abc_registry)
+        subclasses = set()
+        def recurse_abc(c):
+            # recurse into ABC's subclasses
+            subclasses.update(c._abc_registry)
+            for sc in c.__subclasses__():
+                recurse_abc(sc)
+        def recurse_subclasses(c):
+            # recurse into registered virtual classes
+            subclasses.add(c)
+            for sc in c.__subclasses__():
+                recurse_subclasses(sc)
+        recurse_abc(cls)
+        for c in subclasses.copy():
+            recurse_subclasses(c)
+        return subclasses
+
     def _dump_registry(cls, file=None):
         """Debug helper to print the ABC registry."""
         print("Class: %s.%s" % (cls.__module__, cls.__name__), file=file)
diff -r b0cc8c9ab912 Lib/test/test_abc.py
--- a/Lib/test/test_abc.py	Tue Aug 13 19:51:29 2013 -0400
+++ b/Lib/test/test_abc.py	Wed Aug 14 15:12:54 2013 +0200
@@ -258,12 +258,16 @@
         self.assertFalse(issubclass(B, (A,)))
         self.assertNotIsInstance(b, A)
         self.assertNotIsInstance(b, (A,))
+        self.assertEqual(A.get_virtual_classes(), set())
+        self.assertEqual(A.get_virtual_classes(1), set())
         B1 = A.register(B)
         self.assertTrue(issubclass(B, A))
         self.assertTrue(issubclass(B, (A,)))
         self.assertIsInstance(b, A)
         self.assertIsInstance(b, (A,))
         self.assertIs(B1, B)
+        self.assertEqual(A.get_virtual_classes(), {B})
+        self.assertEqual(A.get_virtual_classes(1), {B})
         class C(B):
             pass
         c = C()
@@ -271,6 +275,27 @@
         self.assertTrue(issubclass(C, (A,)))
         self.assertIsInstance(c, A)
         self.assertIsInstance(c, (A,))
+        self.assertEqual(A.get_virtual_classes(), {B})
+        self.assertEqual(A.get_virtual_classes(1), {B, C})
+        class A2(A):
+            pass
+        class D:
+            pass
+        self.assertEqual(A.get_virtual_classes(), {B})
+        self.assertEqual(A.get_virtual_classes(1), {B, C})
+        self.assertEqual(A2.get_virtual_classes(), set())
+        self.assertEqual(A2.get_virtual_classes(1), set())
+        A2.register(D)
+        self.assertEqual(A.get_virtual_classes(), {B})
+        self.assertEqual(A.get_virtual_classes(1), {B, C, D})
+        self.assertEqual(A2.get_virtual_classes(), {D})
+        self.assertEqual(A2.get_virtual_classes(1), {D})
+        class E(D):
+            pass
+        self.assertEqual(A.get_virtual_classes(), {B})
+        self.assertEqual(A.get_virtual_classes(1), {B, C, D, E})
+        self.assertEqual(A2.get_virtual_classes(), {D})
+        self.assertEqual(A2.get_virtual_classes(1), {D, E})
 
     def test_register_as_class_deco(self):
         class A(metaclass=abc.ABCMeta):