diff -r 3fbfa61634de Lib/unittest/loader.py
--- a/Lib/unittest/loader.py	Thu Jul 19 00:14:35 2012 -0500
+++ b/Lib/unittest/loader.py	Fri Jul 20 15:32:39 2012 -0300
@@ -170,6 +170,8 @@
         packages can continue discovery themselves. top_level_dir is stored so
         load_tests does not need to pass this argument in to loader.discover().
         """
+        previous_top_level_dir = self._top_level_dir
+
         set_implicit_top = False
         if top_level_dir is None and self._top_level_dir is not None:
             # make top_level_dir optional if called from load_tests in a package
@@ -211,6 +213,7 @@
             raise ImportError('Start directory is not importable: %r' % start_dir)
 
         tests = list(self._find_tests(start_dir, pattern))
+        self._top_level_dir = previous_top_level_dir
         return self.suiteClass(tests)
 
     def _get_directory_containing_module(self, module_name):
diff -r 3fbfa61634de Lib/unittest/test/test_discovery.py
--- a/Lib/unittest/test/test_discovery.py	Thu Jul 19 00:14:35 2012 -0500
+++ b/Lib/unittest/test/test_discovery.py	Fri Jul 20 15:32:39 2012 -0300
@@ -180,10 +180,50 @@
         top_level_dir = os.path.abspath('/foo/bar')
         start_dir = os.path.abspath('/foo/bar/baz')
         self.assertEqual(suite, "['tests']")
-        self.assertEqual(loader._top_level_dir, top_level_dir)
         self.assertEqual(_find_tests_args, [(start_dir, 'pattern')])
         self.assertIn(top_level_dir, sys.path)
 
+    def test_discover_should_not_persist_top_level_dir_between_calls(self):
+        loader = unittest.TestLoader()
+
+        original_isfile = os.path.isfile
+        def restore_isfile():
+            os.path.isfile = original_isfile
+
+        os.path.isfile = lambda path: True
+        self.addCleanup(restore_isfile)
+
+        original_isdir = os.path.isdir
+        def restore_isdir():
+            os.path.isdir = original_isdir
+
+        os.path.isdir = lambda path: True
+        self.addCleanup(restore_isdir)
+
+        orig_sys_path = sys.path[:]
+        def restore_path():
+            sys.path[:] = orig_sys_path
+        self.addCleanup(restore_path)
+
+        def _find_tests(start_dir, pattern):
+            return ['tests']
+        loader._find_tests = _find_tests
+        loader.suiteClass = str
+
+        loader.discover('/first/one/dir', 'pattern', '/first/one')
+        loader.discover('/second/two/dir', 'pattern', '/second/two')
+
+        # restores to default
+        self.assertEqual(loader._top_level_dir, None)
+
+        loader._top_level_dir = '/previous/dir'
+
+        loader.discover('/first/one/dir', 'pattern', '/first/one')
+        loader.discover('/second/two/dir', 'pattern', '/second/two')
+
+        # restores to previous value
+        self.assertEqual(loader._top_level_dir, '/previous/dir')
+
     def test_discover_with_modules_that_fail_to_import(self):
         loader = unittest.TestLoader()