From February 1976 to May 1981 at the University of Ottawa Heart Institute, 137 mitral valves were replaced with the Hancock porcine bioprosthesis. This study concerns 111 patients (55 men, 56 women) who had single-valve replacement. The average age of the patients was 53.0 years. The predominant lesion in the 102 patients who were receiving their first substitute valve was stenosis in 46% and regurgitation in 42%. Preoperatively, 82% were New York Heart Association (NYHA) class III or IV. Additional procedures were performed in 44 patients (39.6%). The 26 patients who had multiple valves replaced were considered only for durability studies. Hospital mortality was 8.9% for isolated replacement and 22% for replacement associated with aortocoronary bypass grafting. At the time of discharge, 50% of patients were in atrial fibrillation, 14% were taking warfarin and 82% antiplatelet agents. Follow-up was available for a total of 374 patient-years (average of 5.4 years per patient). The mean symptomatic improvement was 1.6 NYHA classes per patient. Actuarial analysis indicated a survival rate of 70% at 8 years. Of the 18 late deaths, 4 (22%) were valve-related. The rates of endocarditis and thromboembolism were 1.1% and 5.8% patient-year respectively. In the 137 prostheses at risk, intrinsic failure occurred at the rate of 4.7% patient-year for a cumulative rate of 35% at 8 years. The authors conclude that the Hancock porcine bioprosthesis provides good relief of symptoms in the mitral position with an acceptable rate of thromboembolism even without anticoagulation.(ABSTRACT TRUNCATED AT 250 WORDS)