diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -1881,10 +1881,14 @@
     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 +1994,11 @@
                 ctx = change(rev)
                 if not fns:
                     def fns_generator():
-                        for f in ctx.files():
+                        if allfiles and len(revs) == 1:
+                            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
@@ -2408,6 +2408,8 @@
     ('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-grep.t b/tests/test-grep.t
--- a/tests/test-grep.t
+++ b/tests/test-grep.t
@@ -368,3 +368,20 @@
   binfile.bin:0:+: Binary file matches
 
   $ 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 ..