diff --git a/xdoc/xdocs/reference/xdocs.xml b/xdoc/xdocs/reference/xdocs.xml
index dcf5b23c..cc4e09ee 100644
--- a/xdoc/xdocs/reference/xdocs.xml
+++ b/xdoc/xdocs/reference/xdocs.xml
@@ -142,6 +142,16 @@
         source files are valid xdoc documents. This should ensure that the
         generated html files are valid
         <a href="http://www.w3.org/TR/xhtml1/">XHTML1-transitional</a>.
+      </p>
+      <p>
+        To support validation of your xdoc source files by an external tool,
+        you should include the following <code>DOCTYPE</code>
+        declaration just after the <code>&lt;xml&gt;</code> element:
+      </p>
+      <source>&lt;!DOCTYPE xdoc PUBLIC
+  "-//Apache Software Foundation//DTD XDOC 1.0//EN"
+  "http://www.apache.org/dtds/xdoc_1_0.dtd"&gt;</source>
+      <p>
         Here is a list of common mistakes to be aware of:
       </p>