Dipyridamole is a vasodilator known to reduce platelet adhesiveness and aggregation. Either dipyridamole or a placebo was given in a daily dose of 400 mg as a random, blind trial to 100 patients who had undergone prosthetic cardiac valve replacement. Patients in both groups were anticoagulated with warfarin sodium. Fifty patients of the placebo group followed for 695 months had 18% incidence of systemic arterial embolism. Forty-two patients in the dipyridamole group were followed for 553 months. One patient exhibited clinical evidence of arterial embolism. Dipyridamole was discontinued shortly after surgery in eight patients followed for 119 months. Two of these patients developed cerebral emboli six months after discontinuation of treatment. It is tentatively concluded that the addition of dipyridamole to a program of anticoagulation reduces the incidence of postoperative arterial emboli originating on prosthetic cardiac valves in those patients who can tolerate the drug.