Ninety-nine patients underwent surgery for acquired diseases of the mitral and aortic valves between August 22, 1988 and August 21, 1990, of these, 64 procedures were done on the aortic valve and 35 procedures on the mitral valve. The early mortality for mitral valve surgery was 5.8% and late death occurred at a rate of 8.8%. As for the aortic valve surgery the operative mortality was zero and late death occurred at a rate of 3.4%. Thromboembolism occurred at a rate of 3.3% per patient year for patient undergoing mitral valve surgery and 6.3% per patient year for those undergoing aortic valve surgery. Valve thrombosis occurred at a rate of 6.6% per patient year for patients undergoing mitral valve surgery and zero for patients undergoing aortic valve surgery. Anticoagulation related bleeding occurred at a rate of 49% per patient year for patients undergoing mitral valve surgery and fatal bleeding at a rate of 3.3% per patient year. For those patients undergoing aortic valve surgery bleeding occurred at a rate of 25% per patient year and fatal bleeding occurred at a rate of 3.1%. This rate of bleeding appears high; and is due to poorly monitored anticoagulation.