Eight hundred twenty-four patients who had cardiac valve replacement with a porcine bioprosthesis were given either Coumadin (sodium warfarin) or aspirin. No patient in sinus rhythm no matter what valve or valves were replaced had thromboembolism whether treated with Coumadin (124 patients) or aspirin (260 patients). One hundred fifty-one patients who had mitral valve replacement, were in atrial fibrillation, and were treated with Coumadin had seven embolic events (4.6%), while 135 similar patients treated with aspirin had five embolic events (3.7%). Fifty-one patients who had double-valve replacement, were in atrial fibrillation, and were treated with coumadin had three embolic episodes (5.9%), while 86 similar patients treated with aspirin had three embolic events (3.5%). No statistical difference was found in the incidence of thromboembolism between similar groups of patients whether treated with Coumadin or aspirin (p greater than 0.05). Hemorrhagic complications were higher and reached statistical significance (p less than 0.001) for the group treated with Coumadin. This study shows that aspirin prevents thromboembolic complications as well as Coumadin in patients having cardiac valve replacement with a bioprosthesis, and results in a lower rate of complications.