Index: category-template.php
===================================================================
--- category-template.php	(revision 25008)
+++ category-template.php	(working copy)
@@ -871,6 +871,11 @@
 			$class = 'cat-item cat-item-' . $category->term_id;
 			if ( !empty($current_category) ) {
 				$_current_category = get_term( $current_category, $category->taxonomy );
+				$category_ancestors = array_reverse(get_ancestors($current_category, $category->taxonomy));
+				
+				if (in_array($category->term_id, $category_ancestors) && $hierarchical)
+					$class .= ' current-cat-ancestor';
+				
 				if ( $category->term_id == $current_category )
 					$class .=  ' current-cat';
 				elseif ( $category->term_id == $_current_category->parent )