To determine the clinical significance of left ventricular thrombi, we used two-dimensional echocardiography to study 261 patients with acute transmural myocardial infarction. Mural thrombi were found in 46 patients. This complication occurred in 34% (44 of 130) of anterior wall infarctions but in only 1.5% (2 of 131) of inferior wall infarctions. An apical wall motion abnormality was present in all patients with thrombus. Severe depression of left ventricular function was not a prerequisite for thrombus formation: the mean left ventricular ejection fraction was 37 +/- 1.5%. Forty-three patients with left ventricular thrombi were followed for a mean duration of 15 months with serial echocardiography. None of the 25 patients who received anticoagulation treatment had an embolic event. Embolization occurred in 7 of 18 patients who had not received anticoagulation treatment. All embolic events occurred within 4 months of infarction. Although anticoagulation treatment appeared to provide protection against embolic events, the prevalence of left ventricular thrombi on follow-up echocardiographic study was essentially the same whether or not this treatment was used.