The incidence of thromboembolism after insertion of prosthetic cardiac valves is related to the type of prosthesis and in the cases reviewed here was highest for the ball-valve prosthesis. The risk of thromboembolism was studied by constructing actuarial curves showing the proportion of patients with embolism at increasing time intervals postoperatively. Such curves for the patients studied suggest that the risk of thromboembolism diminishes with time, particularly by the third postoperative year. The risk was remarkably similar for patients having aortic, mitral, and multiplevalve replacements, being 15% for the first 6 postoperative months, another 8% for the next 6 months, and another 20% for the next 2 years. Emboli lodged in cerebral, coronary, renal, and other sites in these patients in diminishing frequency. The risk of fatality averaged 15%. None of the nonfatal embolisms resulted in significant continuing disability, save for 7% of the cerebral embolisms. A beneficial effect of anticoagulation was demonstrated for patients having replacement of the aortic valve with a ball-valve prosthesis, particularly when this treatment was carefully controlled. In the latter group, the risk of thromboembolism was less than 5% 2 years postoperatively.