Gross anatomic, histologic and ultrastructural observations were made on bovine pericardial bioprosthetic cardiac valves removed because of functional failure four to eight years after implantation in patients. Changes observed included: primary tissue failure associated with the use of an alignment suture, thickening of valve leaflet, leaflet tissue delamination, leaflet calcification, and dystrophic alterations of collagen. It is concluded that structural failure of pericardial valves can result from problems of design, particularly the use of alignment sutures, and from tissue factors peculiar to pericardium.