To determine the incidence and natural history of left ventricular thrombosis in acute transmural myocardial infarction we performed serial two-dimensional echocardiography in 51 patients. Seventeen patients had inferior infarcts. None of these developed left ventricular thrombosis. The remaining 34 patients had anterior infarcts. Ten of these developed left ventricular thrombus at an average of 4 +/- 2 days after admission. All patients with left ventricular thrombosis had apical akinesia or dyskinesia. Patients with anterior myocardial infarction and akinesia or dyskinesia of the apex are at high risk of developing left ventricular thrombosis. Peak aspartate aminotransferase and lactate dehydrogenase enzyme activity were of little value in identifying this high risk group.