diff -r c7a700132018 Lib/os.py
--- a/Lib/os.py	Thu Mar 12 10:32:20 2015 +0100
+++ b/Lib/os.py	Thu Mar 12 10:39:19 2015 +0100
@@ -354,7 +354,7 @@ def walk(top, topdown=True, onerror=None
 
     dirs = []
     nondirs = []
-    symlinks = set()
+    symlinks = {}
 
     # We may not have read permission for top, in which case we can't
     # get a list of the files the directory contains.  os.walk
@@ -379,13 +379,12 @@ def walk(top, topdown=True, onerror=None
 
             if is_dir and not followlinks:
                 try:
-                    if entry.is_symlink():
-                        symlinks.add(entry.name)
+                    symlinks[entry.name] = entry.is_symlink()
                 except OSError:
                     # If is_symlink() raises an OSError, consider that the
                     # entry is not a symbolik link, same behaviour than
                     # os.path.islink().
-                    pass
+                    symlinks[entry.name] = False
     except OSError as error:
         # scandir() or iterating into scandir() iterator raised an OSError
         if onerror is not None:
@@ -398,7 +397,15 @@ def walk(top, topdown=True, onerror=None
 
     # Recurse into sub-directories
     for name in dirs:
-        if followlinks or name not in symlinks:
+        if followlinks:
+            walk_into = True
+        else:
+            try:
+                walk_into = not symlinks[name]
+            except KeyError:
+                # directory added after dirs was created
+                walk_into = not path.islink(path.join(top, name))
+        if walk_into:
             new_path = path.join(top, name)
             yield from walk(new_path, topdown, onerror, followlinks)
 
diff -r c7a700132018 Lib/test/test_os.py
--- a/Lib/test/test_os.py	Thu Mar 12 10:32:20 2015 +0100
+++ b/Lib/test/test_os.py	Thu Mar 12 10:39:19 2015 +0100
@@ -884,6 +884,41 @@ class WalkTests(unittest.TestCase):
         else:
             self.fail("Didn't follow symlink with followlinks=True")
 
+    def check_add_dir(self, create_symlink, follow_symlinks):
+        if create_symlink:
+            dir2 = os.path.realpath(support.TESTFN + "2")
+            os.mkdir(dir2)
+            self.addCleanup(os.rmdir, dir2)
+
+        newdir = os.path.join(self.walk_path, "newdir")
+
+        walk_it = self.walk(self.walk_path, topdown=True,
+                            follow_symlinks=follow_symlinks)
+        seen = False
+        for root, dirs, files in walk_it:
+            if root == self.walk_path:
+                if create_symlink:
+                    os.symlink(dir2, newdir)
+                else:
+                    os.mkdir(newdir)
+                dirs.append(os.path.basename(newdir))
+            elif root == newdir:
+                seen = True
+                break
+        self.assertEqual(seen, follow_symlinks or not create_symlink)
+
+    def test_add_dir(self):
+        self.check_add_dir(create_symlink=False, follow_symlinks=False)
+
+    def test_add_dir_symlink(self):
+        self.check_add_dir(create_symlink=True, follow_symlinks=False)
+
+    def test_add_dir_follow(self):
+        self.check_add_dir(create_symlink=False, follow_symlinks=True)
+
+    def test_add_dir_symlink_follow(self):
+        self.check_add_dir(create_symlink=True, follow_symlinks=True)
+
     def tearDown(self):
         # Tear everything down.  This is a decent use for bottom-up on
         # Windows, which doesn't have a recursive delete command.  The