In a prospective study of 90 consecutive patients with acute myocardial infarction, 15 (28.3%) of 53 patients with an anterior wall infarction developed a left ventricular thrombosis diagnosed by cross sectional echocardiography. Patients received anticoagulants only after a left ventricular thrombosis had been diagnosed. Twenty eight patients had an inferior infarction, but none of these had a left ventricular thrombosis. Five (5.5%) of the 90 patients suffered a cerebrovascular accident, and all had an anterior wall infarction. In four of these five patients a left ventricular thrombosis was confirmed by echocardiography before the cerebrovascular accident. All patients with left ventricular thrombosis had apical akinesis. The incidence of a thrombosis did not differ in patients with a first anterior myocardial infarction and with reinfarctions. Among the 40 patients with a first anterior wall infarction, 12 with a thrombosis had a significantly higher incidence of enlarged heart on chest radiographs and significantly higher serum aspartate aminotransferase enzyme activity than those without. Thus patients with a large anterior wall infarction and with akinesis in the apical region are at high risk of developing a left ventricular thrombosis, which may be a source of peripheral emboli. Left ventricular thrombosis appears to be rare with infarctions in other locations.