Two hundred fourteen survivors of single aortic (AVR) or mitral valve replacement (MVR) were evaluated. The present model of the Smeloff-Cutter prosthesis was used in these patients, and the series was started in September, 1966, following the last structural change in the valve. Clinical follow-up ended in September, 1976. Bleeding, thromboembolism, and peristent left ventricular dysfunction were the major complications. Thromboembolism occurred at a rate of 0.13 percent per month of patient follow-up. Late deaths occurred in 19.2 percent of patients, half of these within the first year. Acturarial data indicated a 5 year survival rate of approximately 75 percent after both mitral and aortic replacements. Bleeding and thromboembolism were more frequent causes of death after mitral replacement. Myocardial function was of greatest importance in long-term survival after replacement of either valve. Variations in warfarin dosage significantly affected both bleeding and thromboembolic complications.