diff --git a/reference/mongo/mongocollection/ensureindex.xml b/reference/mongo/mongocollection/ensureindex.xml
index c1e4240e4d..c73a030ce2 100644
--- a/reference/mongo/mongocollection/ensureindex.xml
+++ b/reference/mongo/mongocollection/ensureindex.xml
@@ -80,11 +80,20 @@
           <literal>"background"</literal>
          </para>
          <para>
-          If you are using MongoDB version 1.3.2+, you can create indexes in the
-          background while other operations are taking place.  By default, index
-          creation happens synchronously.  If you specify &true; with this 
-          option, index creation will be asynchronous.
+          By default, index creation is a blocking operation and will stop other
+          operations on the database from proceeding until completed. If you
+          specify &true; for this option, the index will be created in the
+          background while other operations are taking place.
          </para>
+         <warning>
+          <para>
+           Prior to MongoDB 2.1.0, the index build operation is not a background
+           build when it replicates to secondaries, irrespective of this option.
+           See
+           <link xlink:href="&url.mongodb.dochub.indexes.rs;">Building Indexes with Replica Sets</link>
+           for more information.
+          </para>
+         </warning>
         </listitem>
         <listitem>
          <para>