diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -1881,10 +1881,13 @@
     yielding each context, the iterator will first call the prepare
     function on each context in the window in forward order.'''
 
+    allfiles = opts.get('allfiles')
     follow = opts.get('follow') or opts.get('follow_first')
     revs = _walkrevs(repo, opts)
     if not revs:
         return []
+    if allfiles and len(revs) > 1:
+        raise error.Abort(_("multiple revisions not supported with --allfiles"))
     wanted = set()
     slowpath = match.anypats() or (not match.always() and opts.get('removed'))
     fncache = {}
@@ -1990,7 +1993,11 @@
                 ctx = change(rev)
                 if not fns:
                     def fns_generator():
-                        for f in ctx.files():
+                        if allfiles:
+                            fiter = iter(ctx)
+                        else:
+                            fiter = ctx.files()
+                        for f in fiter:
                             if match(f):
                                 yield f
                     fns = fns_generator()
diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2403,6 +2403,8 @@
     ('n', 'line-number', None, _('print matching line numbers')),
     ('r', 'rev', [],
      _('only search files changed within revision range'), _('REV')),
+    ('', 'allfiles', False,
+     _('include all files in the changeset while grepping (EXPERIMENTAL)')),
     ('u', 'user', None, _('list the author (long with -v)')),
     ('d', 'date', None, _('list the date (short with -q)')),
     ] + formatteropts + walkopts,
diff --git a/tests/test-completion.t b/tests/test-completion.t
--- a/tests/test-completion.t
+++ b/tests/test-completion.t
@@ -313,7 +313,7 @@
   debugwireproto: localssh, peer, noreadstderr, nologhandshake, ssh, remotecmd, insecure
   files: rev, print0, include, exclude, template, subrepos
   graft: rev, continue, stop, edit, log, force, currentdate, currentuser, date, user, tool, dry-run
-  grep: print0, all, text, follow, ignore-case, files-with-matches, line-number, rev, user, date, template, include, exclude
+  grep: print0, all, text, follow, ignore-case, files-with-matches, line-number, rev, allfiles, user, date, template, include, exclude
   heads: rev, topo, active, closed, style, template
   help: extension, command, keyword, system
   identify: rev, num, id, branch, tags, bookmarks, ssh, remotecmd, insecure, template
diff --git a/tests/test-grep.t b/tests/test-grep.t
--- a/tests/test-grep.t
+++ b/tests/test-grep.t
@@ -372,6 +372,23 @@
 
   $ cd ..
 
+Test for showing working of allfiles flag
+
+  $ hg init sng
+  $ cd sng
+  $ echo "unmod" >> um
+  $ hg ci -A -m "adds unmod to um"
+  adding um
+  $ echo "something else" >> new
+  $ hg ci -A -m "second commit"
+  adding new
+  $ hg grep -r "." "unmod"
+  [1]
+  $ hg grep -r "." "unmod" --allfiles
+  um:1:unmod
+
+  $ cd ..
+
 Fix_Wdir(): test that passing wdir() t -r flag does greps on the
 files modified in the working directory