diff -r d5536c06a082 Lib/ntpath.py
--- a/Lib/ntpath.py	Fri Jul 05 01:40:52 2013 +0200
+++ b/Lib/ntpath.py	Sun Jul 07 00:03:45 2013 +0200
@@ -620,6 +620,11 @@
     start_list = [x for x in start_rest.split(sep) if x]
     path_list = [x for x in path_rest.split(sep) if x]
     # Work out how much of the filepath is shared by start and path.
+
+    if start_list and "." in str(start_list[-1]):
+        # A file (not directory) is given as an argument for 'start'.
+        start_list = start_list[:-1]
+
     i = 0
     for e1, e2 in zip(start_list, path_list):
         if normcase(e1) != normcase(e2):
diff -r d5536c06a082 Lib/posixpath.py
--- a/Lib/posixpath.py	Fri Jul 05 01:40:52 2013 +0200
+++ b/Lib/posixpath.py	Sun Jul 07 00:03:45 2013 +0200
@@ -442,6 +442,10 @@
     start_list = [x for x in abspath(start).split(sep) if x]
     path_list = [x for x in abspath(path).split(sep) if x]
 
+    if start_list and "." in str(start_list[-1]):
+        # A file (not directory) is given as an argument for 'start'.
+        start_list = start_list[:-1]
+
     # Work out how much of the filepath is shared by start and path.
     i = len(commonprefix([start_list, path_list]))
 
diff -r d5536c06a082 Lib/test/test_ntpath.py
--- a/Lib/test/test_ntpath.py	Fri Jul 05 01:40:52 2013 +0200
+++ b/Lib/test/test_ntpath.py	Sun Jul 07 00:03:45 2013 +0200
@@ -242,6 +242,7 @@
         tester('ntpath.relpath("/a", "/a")', '.')
         tester('ntpath.relpath("/a/b", "/a/b")', '.')
         tester('ntpath.relpath("c:/foo", "C:/FOO")', '.')
+        tester('ntpath.relpath("bar/bat", "foo.ext")', 'bar\\bat')
 
     def test_sameopenfile(self):
         with TemporaryFile() as tf1, TemporaryFile() as tf2:
diff -r d5536c06a082 Lib/test/test_posixpath.py
--- a/Lib/test/test_posixpath.py	Fri Jul 05 01:40:52 2013 +0200
+++ b/Lib/test/test_posixpath.py	Sun Jul 07 00:03:45 2013 +0200
@@ -505,6 +505,7 @@
             self.assertEqual(posixpath.relpath("/", "/"), '.')
             self.assertEqual(posixpath.relpath("/a", "/a"), '.')
             self.assertEqual(posixpath.relpath("/a/b", "/a/b"), '.')
+            self.assertEqual(posixpath.relpath("foo/bar", "bat.ext"), "foo/bar")
         finally:
             os.getcwd = real_getcwd
 
@@ -532,6 +533,8 @@
             self.assertEqual(posixpath.relpath(b"/", b"/"), b'.')
             self.assertEqual(posixpath.relpath(b"/a", b"/a"), b'.')
             self.assertEqual(posixpath.relpath(b"/a/b", b"/a/b"), b'.')
+            self.assertEqual(posixpath.relpath(b"foo/bar", b"bat.ext"), b"foo/bar")
+
 
             self.assertRaises(TypeError, posixpath.relpath, b"bytes", "str")
             self.assertRaises(TypeError, posixpath.relpath, "str", b"bytes")