To study the incidence of left-ventricular thrombosis after transmural myocardial infarction, we performed serial two-dimensional echocardiography in 70 consecutive patients. Thirty-five patients had inferior-wall infarction: none had a left-ventricular thrombus. The other 35 had anterior-wall infarction: 12 had left-ventricular thrombi. Thrombi were diagnosed an average of five days after the infarction (range, one to 11 days). All patients with left-ventricular thrombi had severe apical-wall-motion abnormalities (akinesis or dyskinesis). Twenty-six of the 35 patients with anterior infarctions had apical akinesis or dyskinesis on echocardiography; left-ventricular thrombi developed in 12 of these 26 (46 per cent). We conclude that patients with severe apical-wall-motion abnormalities during acute transmural anterior myocardial infarction are at high risk for left-ventricular thrombosis. This high-risk group can be identified before the development of left-ventricular thrombi. Patients with inferior infarction or anterior infarction without a severe apical-wall-motion abnormality are at low risk. (N Engl J Med. 1981; 305:297–302.) STUDIES of left-ventricular thrombosis as a complication of acute myocardial infarction have largely been limited to autopsy series, since no practical noninvasive method to detect a left-ventricular thrombus reliably has existed. Hence, little information is available on the incidence of left-ventricular thrombi in patients surviving acute myocardial infarction. Recent reports indicate that two-dimensional echocardiography is a reliable noninvasive technique for the detection of left-ventricular thrombi.1 2 3 4 5 Using serial two-dimensional echocardiography, we prospectively studied a consecutive group of patients with acute transmural myocardial infarction to determine the incidence of left-ventricular thrombosis during acute transmural myocardial infarction, the period in which left-ventricular thrombi.