diff --git a/ndiff/docs/ndiff.1 b/ndiff/docs/ndiff.1
index 1ab083471..aedbdbb56 100644
--- a/ndiff/docs/ndiff.1
+++ b/ndiff/docs/ndiff.1
@@ -2,12 +2,12 @@
 .\"     Title: ndiff
 .\"    Author: [see the "Authors" section]
 .\" Generator: DocBook XSL Stylesheets v1.74.3 <http://docbook.sf.net/>
-.\"      Date: 07/22/2009
+.\"      Date: 07/30/2009
 .\"    Manual: User Commands
 .\"    Source: Ndiff
 .\"  Language: English
 .\"
-.TH "NDIFF" "1" "07/22/2009" "Ndiff" "User Commands"
+.TH "NDIFF" "1" "07/30/2009" "Ndiff" "User Commands"
 .\" -----------------------------------------------------------------
 .\" * set default formatting
 .\" -----------------------------------------------------------------
@@ -370,6 +370,44 @@ If the script is saved as
 .RE
 .\}
 .sp
+.SH "EXIT CODE"
+.PP
+The exit code indicates whether the scans are equal\&.
+.sp
+.RS 4
+.ie n \{\
+\h'-04'\(bu\h'+03'\c
+.\}
+.el \{\
+.sp -1
+.IP \(bu 2.3
+.\}
+0 means that the scans are the same in all the aspects Ndiff knows about\&.
+.RE
+.sp
+.RS 4
+.ie n \{\
+\h'-04'\(bu\h'+03'\c
+.\}
+.el \{\
+.sp -1
+.IP \(bu 2.3
+.\}
+1 means that the scans differ\&.
+.RE
+.sp
+.RS 4
+.ie n \{\
+\h'-04'\(bu\h'+03'\c
+.\}
+.el \{\
+.sp -1
+.IP \(bu 2.3
+.\}
+2 indicates a runtime error, such as the failure to open a file\&.
+.RE
+.sp
+.RE
 .SH "BUGS"
 .PP
 Report bugs to the
diff --git a/ndiff/docs/ndiff.xml b/ndiff/docs/ndiff.xml
index 816eec4a0..afe844999 100644
--- a/ndiff/docs/ndiff.xml
+++ b/ndiff/docs/ndiff.xml
@@ -355,6 +355,21 @@ ln -sf scan-$date.xml scan-prev.xml
     </para>
   </refsect1>
 
+  <refsect1>
+    <title>Exit Code</title>
+
+    <para>
+    The exit code indicates whether the scans are equal.
+    <itemizedlist spacing="compact">
+      <listitem><para>0 means that the scans are the same in all the
+      aspects Ndiff knows about.</para></listitem>
+      <listitem><para>1 means that the scans differ.</para></listitem>
+      <listitem><para>2 indicates a runtime error, such as the failure
+      to open a file.</para></listitem>
+    </itemizedlist>
+    </para>
+  </refsect1>
+
   <refsect1>
     <title>Bugs</title>
     <para>
diff --git a/ndiff/ndiff b/ndiff/ndiff
index 466ef7981..277052edc 100755
--- a/ndiff/ndiff
+++ b/ndiff/ndiff
@@ -446,6 +446,8 @@ class ScanDiff(object):
         document.writexml(f, addindent = u"  ", newl = u"\n", encoding = "UTF-8")
         document.unlink()
 
+    cost = property(lambda self: sum([hd.cost for hd in self.host_diffs.values()]))
+
 class HostDiff(object):
     """A diff of two Hosts. It contains the two hosts, variables describing what
     changed, and a list of PortDiffs and OS differences."""
@@ -1150,10 +1152,14 @@ service and OS detection.
   --xml          display output in XML format\
 """ % sys.argv[0]
 
+EXIT_EQUAL = 0
+EXIT_DIFFERENT = 1
+EXIT_ERROR = 2
+
 def usage_error(msg):
     print >> sys.stderr, u"%s: %s" % (sys.argv[0], msg)
     print >> sys.stderr, u"Try '%s -h' for help." % sys.argv[0]
-    sys.exit(1)
+    sys.exit(EXIT_ERROR)
 
 def main():
     global verbose
@@ -1199,5 +1205,17 @@ def main():
     elif output_format == "xml":
         diff.print_xml()
 
+    if diff.cost == 0:
+        return EXIT_EQUAL
+    else:
+        return EXIT_DIFFERENT
+
+# Catch uncaught exceptions so they can produce an exit code of 2 (EXIT_ERROR),
+# not 1 like they would by default.
+def excepthook(type, value, tb):
+    sys.__excepthook__(type, value, tb)
+    sys.exit(EXIT_ERROR)
+
 if __name__ == "__main__":
-    main()
+    sys.excepthook = excepthook
+    sys.exit(main())
diff --git a/ndiff/ndifftest.py b/ndiff/ndifftest.py
index ed47bd69f..c6a08e60c 100755
--- a/ndiff/ndifftest.py
+++ b/ndiff/ndifftest.py
@@ -2,6 +2,7 @@
 
 # Unit tests for Ndiff.
 
+import subprocess
 import unittest
 import xml.dom.minidom
 import StringIO
@@ -688,4 +689,43 @@ def host_apply_diff(host, diff):
             host.script_results[host.script_results.index(sr_a)] = sr_b
     host.script_results.sort()
 
+def call_quiet(args, **kwargs):
+    """Run a command with subprocess.call and hide its output."""
+    return subprocess.call(args, stdout = subprocess.PIPE,
+        stderr = subprocess.STDOUT, **kwargs)
+
+class exit_code_test(unittest.TestCase):
+    NDIFF = "./ndiff"
+
+    def test_exit_equal(self):
+        """Test that the exit code is 0 when the diff is empty."""
+        for format in ("--text", "--xml"):
+            code = call_quiet([self.NDIFF, format, 
+                "test-scans/simple.xml", "test-scans/simple.xml"])
+            self.assertEqual(code, 0)
+        # Should be independent of verbosity.
+        for format in ("--text", "--xml"):
+            code = call_quiet([self.NDIFF, "-v", format, 
+                "test-scans/simple.xml", "test-scans/simple.xml"])
+            self.assertEqual(code, 0)
+
+    def test_exit_different(self):
+        """Test that the exit code is 1 when the diff is not empty."""
+        for format in ("--text", "--xml"):
+            code = call_quiet([self.NDIFF, format, 
+                "test-scans/simple.xml", "test-scans/complex.xml"])
+            self.assertEqual(code, 1)
+
+    def test_exit_error(self):
+        """Test that the exit code is 2 when there is an error."""
+        code = call_quiet([self.NDIFF])
+        self.assertEqual(code, 2)
+        code = call_quiet([self.NDIFF, "test-scans/simple.xml"])
+        self.assertEqual(code, 2)
+        code = call_quiet([self.NDIFF, "test-scans/simple.xml",
+            "test-scans/nonexistent.xml"])
+        self.assertEqual(code, 2)
+        code = call_quiet([self.NDIFF, "--nothing"])
+        self.assertEqual(code, 2)
+
 unittest.main()