diff --git a/accessible/public/nsIAccessible.idl b/accessible/public/nsIAccessible.idl
index ca8c870cc912..4e8b4de50acf 100644
--- a/accessible/public/nsIAccessible.idl
+++ b/accessible/public/nsIAccessible.idl
@@ -186,7 +186,8 @@ interface nsIAccessible : nsISupports
    * labels, radio buttons, etc. Also used for collectons of non-text objects.
    *
    * @param groupLevel - 1-based, similar to ARIA 'level' property
-   * @param similarItemsInGroup - 1-based, similar to ARIA 'setsize' property
+   * @param similarItemsInGroup - 1-based, similar to ARIA 'setsize' property,
+   *                              inclusive of the current item
    * @param positionInGroup - 1-based, similar to ARIA 'posinset' property
    */
   void groupPosition(out long aGroupLevel, out long aSimilarItemsInGroup,
diff --git a/accessible/src/base/nsAccessible.cpp b/accessible/src/base/nsAccessible.cpp
index dac596a0cf17..5043d1c35776 100644
--- a/accessible/src/base/nsAccessible.cpp
+++ b/accessible/src/base/nsAccessible.cpp
@@ -2206,7 +2206,7 @@ nsAccessible::GroupPosition(PRInt32 *aGroupLevel,
   *aGroupLevel = level;
 
   *aPositionInGroup = posInSet;
-  *aSimilarItemsInGroup = setSize - 1;
+  *aSimilarItemsInGroup = setSize;
 
   return NS_OK;
 }