Two-dimensional echocardiography was utilized in a prospective study to find the incidence, characteristics and natural history of left ventricular thrombus in 50 consecutive patients with acute myocardial infarction. The overall incidence of thrombosis was 14%; 19% in anterior and 5% in inferior infarction. The thrombus was detected at the apex in 4, along the ventricular septum in 2 and at both these sites in one patient. It was observed 3-10 days after the infarction. Thrombus at the ventricular septum, seen in this study, has not been reported previously in acute myocardial infarction. Development of apical and septal thrombi was significantly associated with akinesis of these sites. Only 2 patients with protruding thrombi had systemic embolism. We conclude that the apex as well as the septum are common sites for thrombosis in patients with severe wall motion abnormalities following acute anterior infarction. Careful observation of these sites during echocardiography may reveal thrombi prone for embolization.