diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -61,12 +61,12 @@ def dodiff(repo, files = None, node1 = N
         tn = read(f)
         sys.stdout.write(mdiff.unidiff(to, date1, tn, date2, f))
     for f in a:
-        to = ""
+        to = None
         tn = read(f)
         sys.stdout.write(mdiff.unidiff(to, date1, tn, date2, f))
     for f in d:
         to = repo.file(f).read(mmap[f])
-        tn = ""
+        tn = None
         sys.stdout.write(mdiff.unidiff(to, date1, tn, date2, f))
     
 def help(ui, cmd=None):
diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -10,9 +10,23 @@ from mercurial.mpatch import *
 
 def unidiff(a, ad, b, bd, fn):
     if not a and not b: return ""
-    a = a.splitlines(1)
-    b = b.splitlines(1)
-    l = list(difflib.unified_diff(a, b, "a/" + fn, "b/" + fn, ad, bd))
+
+    if a == None:
+        b = b.splitlines(1)
+        l1 = "--- %s\t%s\n" % ("/dev/null", ad)
+        l2 = "+++ %s\t%s\n" % ("b/" + fn, bd)
+        l3 = "@@ -0,0 +1,%d @@\n" % len(b)
+        l = [l1, l2, l3] + ["+" + e for e in b]
+    elif b == None:
+        a = a.splitlines(1)
+        l1 = "--- %s\t%s\n" % ("a/" + fn, ad)
+        l2 = "+++ %s\t%s\n" % ("/dev/null", bd)
+        l3 = "@@ -1,%d +0,0 @@\n" % len(a)
+        l = [l1, l2, l3] + ["-" + e for e in a]
+    else:
+        a = a.splitlines(1)
+        b = b.splitlines(1)
+        l = list(difflib.unified_diff(a, b, "a/" + fn, "b/" + fn, ad, bd))
 
     for ln in xrange(len(l)):
         if l[ln][-1] != '\n':