Index: src/parser/ArcanistDiffParser.php
===================================================================
--- src/parser/ArcanistDiffParser.php
+++ src/parser/ArcanistDiffParser.php
@@ -13,6 +13,7 @@
   protected $lineSaved;
   protected $isGit;
   protected $isMercurial;
+  protected $isRCS;
   protected $detectBinaryFiles = false;
   protected $tryEncoding;
   protected $rawDiff;
@@ -205,6 +206,8 @@
         // This is a git diff, probably from "git show" or "git diff".
         // Note that the filenames may appear quoted.
         '(?P<type>diff --git) (?P<oldnew>.*)',
+        // RCS Diff
+        '(?P<type>rcsdiff -u) (?P<oldnew>.*)',
         // This is a unified diff, probably from "diff -u" or synthetic diffing.
         '(?P<type>---) (?P<old>.+)\s+\d{4}-\d{2}-\d{2}.*',
         '(?P<binary>Binary) files '.
@@ -303,6 +306,10 @@
           $this->setIsMercurial(true);
           $this->parseIndexHunk($change);
           break;
+        case 'rcsdiff -u':
+          $this->isRCS = true;
+          $this->parseIndexHunk($change);
+          break;
         default:
           $this->didFailParse("Unknown diff type.");
           break;
@@ -777,13 +784,30 @@
       $remainder = '\t.*';
     }
 
-    $ok = preg_match(
+    if ($this->isRCS) {
+      $remainder = '\t.*';
+      $this->nextLine();
+      $this->nextLine();
+      $this->nextLine();
+
+      $rcs_pattern = '/^[-+]{3}\s[ab]\/.*'.$remainder.'$/';
+
+      $ok = preg_match($rcs_pattern, $line, $matches);
+      if (!ok) {
+        $this->didFailParse("Not a good Rcs diff or may be rcs parser.");
+      }
+
+    } else {
+      $ok = preg_match(
       '@^[-+]{3} (?:[ab]/)?(?P<path>.*?)'.$remainder.'$@',
       $line,
       $matches);
-    if (!$ok) {
-      $this->didFailParse(
-        "Expected hunk target '+++ path/to/file.ext (revision N)'.");
+
+      if (!$ok) {
+        $this->didFailParse(
+          "Expected hunk target '+++ path/to/file.ext (revision N)'.");
+      }
+
     }
 
     $this->nextLine();
Index: src/parser/__tests__/ArcanistDiffParserTestCase.php
===================================================================
--- src/parser/__tests__/ArcanistDiffParserTestCase.php
+++ src/parser/__tests__/ArcanistDiffParserTestCase.php
@@ -549,6 +549,20 @@
       case 'svn-property-windows.svndiff':
         $this->assertEqual(1, count($changes));
         break;
+      case 'rcs-addline.rcsdiff':
+        $this->assertEqual(1, count($changes));
+        $change = array_shift($changes);
+        $this->assertEqual(
+          ArcanistDiffChangeType::TYPE_CHANGE,
+          $change->getType());
+        break;
+      case 'rcs-deleteline.rcs.diff':
+        $this->assertEqual(1, count($changes));
+        $change = array_shift($changes);
+        $this->assertEqual(
+          ArcanistDiffChangeType::TYPE_CHANGE,
+          $change->getType());
+        break;
       default:
         throw new Exception("No test block for diff file {$diff_file}.");
         break;
Index: src/parser/__tests__/diff/rcs-addline.rcsdiff
===================================================================
--- /dev/null
+++ src/parser/__tests__/diff/rcs-addline.rcsdiff
@@ -0,0 +1,13 @@
+rcsdiff -u a/testing.php
+===================================================================
+RCS file: a/RCS/testing.php,v
+retrieving revision 1.1
+diff -u -r1.1 a/testing.php
+--- a/testing.php      2013/03/11 16:41:39    1.1
++++ a/testing.php      2013/03/11 16:44:10
+@@ -1,3 +1,4 @@
+ <?php
+ echo "Hello World";
++echo "How are you doing today?";
+ ?>
+
Index: src/parser/__tests__/diff/rcs-deleteline.rcsdiff
===================================================================
--- /dev/null
+++ src/parser/__tests__/diff/rcs-deleteline.rcsdiff
@@ -0,0 +1,14 @@
+rcsdiff -u a/testing.php
+===================================================================
+RCS file: a/RCS/testing.php,v
+retrieving revision 1.2
+diff -u -r1.2 a/testing.php
+--- a/testing.php      2013/03/11 16:45:50    1.2
++++ a/testing.php
+2013/03/11 16:46:19
+@@ -1,4 +1,3 @@
+ <?php
+-echo "Hello World";
+ echo "How are you doing today?";
+ ?>
+