diff --git a/mercurial/templatekw.py b/mercurial/templatekw.py
--- a/mercurial/templatekw.py
+++ b/mercurial/templatekw.py
@@ -334,6 +334,19 @@ def showlatesttagdistance(repo, ctx, tem
     """:latesttagdistance: Integer. Longest path to the latest tag."""
     return getlatesttags(repo, ctx, cache)[1]
 
+def showchangessincelatesttag(repo, ctx, templ, cache, **args):
+    """:changessincelatesttag: Integer. All ancestors not in the latest tag."""
+    latesttag = getlatesttags(repo, ctx, cache)[2][0]
+    offset = 0
+    revs = [ctx.rev()]
+
+    # The only() revset doesn't currently support wdir()
+    if ctx.rev() is None:
+        offset = 1
+        revs = [p.rev() for p in ctx.parents()]
+
+    return len(repo.revs('only(%ld, %s)', revs, latesttag)) + offset
+
 def showmanifest(**args):
     repo, ctx, templ = args['repo'], args['ctx'], args['templ']
     mnode = ctx.manifestnode()
@@ -427,6 +440,7 @@ keywords = {
     'branch': showbranch,
     'branches': showbranches,
     'bookmarks': showbookmarks,
+    'changessincelatesttag': showchangessincelatesttag,
     'children': showchildren,
     # currentbookmark is deprecated
     'currentbookmark': showcurrentbookmark,
diff --git a/tests/test-tag.t b/tests/test-tag.t
--- a/tests/test-tag.t
+++ b/tests/test-tag.t
@@ -443,12 +443,25 @@ check that we can merge tags that differ
   $ hg ci -A -m3
   adding f3
   $ hg tag -f t4 t5 t6
+
+  $ hg up -q '.^'
+  $ hg log -r 'wdir()' -T "{changessincelatesttag} changes since {latesttag}\n"
+  1 changes since t4:t5:t6
+  $ hg log -r '.' -T "{changessincelatesttag} changes since {latesttag}\n"
+  0 changes since t4:t5:t6
+  $ echo c5 > f3
+  $ hg log -r 'wdir()' -T "{changessincelatesttag} changes since {latesttag}\n"
+  1 changes since t4:t5:t6
+  $ hg up -qC
+
   $ hg tag --remove t5
   $ echo c4 > f4
-  $ hg log -r '.' -T "{latesttag}\n"
-  t4:t6
+  $ hg log -r '.' -T "{changessincelatesttag} changes since {latesttag}\n"
+  2 changes since t4:t6
   $ hg ci -A -m4
   adding f4
+  $ hg log -r 'wdir()' -T "{changessincelatesttag} changes since {latesttag}\n"
+  4 changes since t4:t6
   $ hg tag t2
   $ hg tag -f t6