The experience of the University of Ottawa Heart Institute with valvular replacement by Ionescu-Shiley bovine pericardial bioprostheses has been recently reviewed. The focus of the present study is to examine the patterns of valve failure in a subgroup of these prostheses, those that required explanation for valve failure caused specifically by cusp wear and tears. A total of 22 aortic and six mitral standard-profile valves failed in this way. Twenty-five of these valves were reassessed and are the subject of this paper. Twenty-eight holes and 56 cusp tears occurred in these valves in locations which, along with coexistent pathologic findings, suggested that the cusp commissural suture/alignment stitch was the point of their origin. The importance of this observation and postulation about the genesis of these cases of valve failure lies in the potential for improvement of this valve's performance by modifications of its manufacture.