In a prospective serial study of 96 patients with acute myocardial infarction, two dimensional echocardiography identified left ventricular thrombus in 18 patients. The majority of thrombi (15) developed within the first 4 days after admission. In three patients thrombi were identified for the first time 4 months after the acute episode. All 18 patients had received therapeutic anticoagulants on admission and had large anterior wall infarctions complicated by severe pump failure and motion abnormalities echocardiographically. None of the patients had systemic embolisation during the study period. Thus, left ventricular thrombus is a not uncommon though silent complication of acute anterior wall infarction even when patients receive therapeutic anticoagulants.