diff --git a/hgext3rd/undo.py b/hgext3rd/undo.py
--- a/hgext3rd/undo.py
+++ b/hgext3rd/undo.py
@@ -192,7 +192,7 @@
     repo.svfs.tryunlink(path)
 
 def _recordnewgap(repo, absoluteindex=None):
-    path = os.path.join('undolog', 'gap')
+    path = 'undolog' + '/' + 'gap'
     if absoluteindex is None:
         rlog = _getrevlog(repo, 'index.i')
         repo.svfs.write(path, str(len(rlog) - 1))
@@ -224,7 +224,7 @@
 def _gapcheck(repo, reverseindex):
     rlog = _getrevlog(repo, 'index.i')
     absoluteindex = _invertindex(rlog, reverseindex)
-    path = os.path.join('undolog', "gap")
+    path = 'undolog' + '/' + 'gap'
     try:
         result = absoluteindex >= int(repo.svfs.read(path))
     except IOError:
@@ -364,10 +364,11 @@
 # Undo:
 
 @command('undo', [
-    ('n', 'index', 1, _("how many steps to undo back")),
-    ('f', 'force', False, _("undo across a gap in memory (ADVANCED)")),
     ('a', 'absolute', False, _("absolute based on command index instead of "
                                "relative undo")),
+    ('f', 'force', False, _("undo across a gap in memory (ADVANCED)")),
+    ('k', 'keep', False, _("keep working copy changes")),
+    ('n', 'index', 1, _("how many steps to undo back")),
 ])
 def undo(ui, repo, *args, **opts):
     """perform an undo
@@ -408,10 +409,20 @@
     repo or another command breaks something.  Undoing across this "gap" can be
     forced, but isn't advised unless its known how the before and after states
     are connected. Undoing up to a gap is safe.
+
+    Use keep to maintain working copy changes.  With keep, undo mimics hg
+    unamend and hg uncommit.  Specifically, files that exsist currently that
+    don't exist at the repo state we are undoing to will remain in your
+    working copy but not in your changeset.  Maintaining your working copy
+    has primarily two downsides: firstly your new working copy won't be clean
+    so you can't simply redo without cleaning your working copy.  Secondly,
+    the operation may be slow if your working copy is large.  If unsure,
+    its generally easier try undo without --keep first and redo if you want
+    to change this.
     """
-
     reverseindex = opts.get("index")
     relativeundo = not opts.get("absolute")
+    keep = opts.get("keep")
 
     with repo.wlock(), repo.lock(), repo.transaction("undo"):
         cmdutil.checkunfinished(repo)
@@ -422,7 +433,7 @@
         if not (opts.get("force") or _gapcheck(repo, reverseindex)):
             raise error.Abort(_("attempted risky undo across"
                                 " missing history"))
-        _undoto(ui, repo, reverseindex)
+        _undoto(ui, repo, reverseindex, keep=keep)
         # store undo data
         # for absolute undos, think of this as a reset
         # for relative undos, think of this as an update
@@ -451,7 +462,7 @@
         _undoto(ui, repo, reverseindex)
         _logundoredoindex(repo, repo.currenttransaction(), reverseindex)
 
-def _undoto(ui, repo, reverseindex):
+def _undoto(ui, repo, reverseindex, keep=False):
     # undo to specific reverseindex
     # requires inhibit extension
     try:
@@ -477,9 +488,34 @@
     # working copy parent
     workingcopyparent = _readnode(repo, "workingparent.i",
                                   nodedict["workingparent"])
-    revealcommits(repo, workingcopyparent)
-    hg.updatetotally(ui, repo, workingcopyparent, workingcopyparent,
-                     clean=False, updatecheck='abort')
+    if not keep:
+        revealcommits(repo, workingcopyparent)
+        hg.updatetotally(ui, repo, workingcopyparent, workingcopyparent,
+                         clean=False, updatecheck='abort')
+    else:
+        # keeps working copy files
+        curctx = repo['.']
+        precnode = bin(workingcopyparent)
+        precctx = repo[precnode]
+
+        changedfiles = []
+        wctx = repo[None]
+        wctxmanifest = wctx.manifest()
+        precctxmanifest = precctx.manifest()
+        dirstate = repo.dirstate
+        diff = precctxmanifest.diff(wctxmanifest)
+        changedfiles.extend(diff.iterkeys())
+
+        with dirstate.parentchange():
+            dirstate.rebuild(precnode, precctxmanifest, changedfiles)
+            # we want added and removed files to be shown
+            # properly, not with ? and ! prefixes
+            for filename, data in diff.iteritems():
+                if data[0][0] is None:
+                    dirstate.add(filename)
+                if data[1][0] is None:
+                    dirstate.remove(filename)
+        obsolete.createmarkers(repo, [(curctx, (precctx,))])
 
     # visible changesets
     addedrevs = revsetlang.formatspec('olddraft(0) - olddraft(%d)',
diff --git a/tests/test-undo.t b/tests/test-undo.t
--- a/tests/test-undo.t
+++ b/tests/test-undo.t
@@ -307,3 +307,13 @@
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg undo -a
   2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+hg undo --keep tests
+  $ touch kfl1 && hg add kfl1
+  $ hg st
+  A kfl1
+  $ hg commit --amend
+  $ hg st
+  $ hg undo --keep
+  $ hg st
+  A kfl1