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;
@@ -734,9 +741,20 @@
       }
     }
 
+    if ($this->isRCS) {
+      // Skip the RCS headers.
+      $this->nextLine();
+      $this->nextLine();
+      $this->nextLine();
+    }
+
     $old_file = $this->parseHunkTarget();
     $new_file = $this->parseHunkTarget();
 
+    if ($this->isRCS) {
+      $change->setCurrentPath($new_file);
+    }
+
     $change->setOldPath($old_file);
 
     $this->parseChangeset($change);
@@ -775,12 +793,15 @@
       // Something like "Fri Aug 26 01:20:50 2005 -0700", don't bother trying
       // to parse it.
       $remainder = '\t.*';
+    } else if ($this->isRCS) {
+      $remainder = '\s.*';
     }
 
     $ok = preg_match(
       '@^[-+]{3} (?:[ab]/)?(?P<path>.*?)'.$remainder.'$@',
       $line,
       $matches);
+
     if (!$ok) {
       $this->didFailParse(
         "Expected hunk target '+++ path/to/file.ext (revision N)'.");
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.rcsdiff':
+        $this->assertEqual(1, count($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 +1,14 @@
+rcsdiff -u a/testing.php
+===================================================================
+RCS file: a/RCS/testing.php,v
+retrieving revision 1.3
+diff -u -r1.3 a/testing.php
+--- a/testing.php      2013/03/14 09:05:54    1.3
++++ a/testing.php      2013/03/14 09:08:45
+@@ -2,4 +2,5 @@
+ echo "Hello World";
+ echo "How are you doing today?";
+ echo "thanks";
++echo "thanks";
+ ?>
+
Index: src/parser/__tests__/diff/rcs-deleteline.rcsdiff
===================================================================
--- /dev/null
+++ src/parser/__tests__/diff/rcs-deleteline.rcsdiff
@@ -0,0 +1,13 @@
+rcsdiff -u a/testing.php
+===================================================================
+RCS file: a/RCS/testing.php,v
+retrieving revision 1.3
+diff -u -r1.3 a/testing.php
+--- a/testing.php      2013/03/14 09:05:54    1.3
++++ a/testing.php      2013/03/14 09:09:18
+@@ -1,5 +1,3 @@
+ <?php
+ echo "Hello World";
+-echo "How are you doing today?";
+-echo "thanks";
+ ?>