Utilizing a questionnaire specifically designed to uncover potential thromboembolic episodes, we personally interviewed 97 patients who underwent valve replacement with the Ionescu-Shiley valve between January, 1977, and June, 1980. In both the aortic and mitral positions, the attrition rate after the first year was low; cumulative survival at 3 to 4 years was 78% (aortic) and 61% (mitral). None of the deaths were attributed to primary valve failure. Patients with aortic prostheses, none of whom were anticoagulated, had a cumulative thromboembolism-free rate of 94% at 3 to 4 years. Patients with mitral prostheses had a thromboembolism-free rate of 71% at 3 to 4 years with 80% of the episodes occurring in patients in atrial fibrillation. Most emboli produced only transient symptoms, and only 1 patient has residual impairment. The Ionescu-Shiley valve performs well in the early years after valve replacement. Anticoagulation should be strongly considered for patients with mitral prostheses who are in atrial fibrillation.