This report provides follow-up data on 557 patients [73 aortic, 357 mitral, and 127 multiple valve replacements] undergone lonescu-Shiley pericardial Xenograft valve replacement at Seoul National University Hospital between January, 1979 and December, 1985. There were 35 early death [6.3%] and 522 operative survivors were observed, and the cumulative follow-up is 1,140 patient-years [mean: 2.18 years per patient] The thromboembolic complications occurred in 34 cases [3.0% per patient-year] and the rate was 2.1% per patient-year for mitral and 0.3% per patient-year for aortic valve replacement in the presence of anticoagulation therapy. Among the 34 embolic episodes, 9 patients were dead [0.8% per patient-year] and the cause of death were 5 cerebral thromboembolism, 2 pulmonary embolism, and 2 intracerebral hemorrhage due to inappropriate anticoagulation after thromboembolic episode. Actuarial probability [+ SEM] of remaining free of thromboembolism for AVR is 88.1 x 11.1% at 5 years, for MVR 79.1 a 13.4% at 7 years and for multiple valve replacement 77.2 e 5.21% at 7 years. The incidence rate of thromboembolic complications after AVR is not less than that of MVR [0.3 Among the potential thromboembolic risk factors, atrial fibrillation is possible risk factor to increase the thromboembolic complication [0.05 < P < 0.1], but the importance of other factors, such as atrial clot, large left atrial size, mitral position, NYHA functional class, and age is less definite. A careful follow-up and the proper control of anticoagulation without omission, poor control, and arbitrary withdrawal is important for the successful management of the thromboembolic complications and the anticoagulation-related morbidity and mortality.