Sixty patients (48 men and 12 women; aged 36 to 72 years, mean 48 +/- 9), who survived an acute anterior myocardial infarction and in whom left ventricular thrombus was detected by cross-sectional echocardiography 1 to 2 days before they were discharged from the hospital, were prospectively studied. All had evidence of left apical wall motion abnormalities. They were randomly divided into three groups of 20 patients each. Group A was given a full dose of oral anticoagulants, group B was given aspirin, 650 mg/day, and group C received no antithrombotic therapy. Echocardiography was performed every 3 months in all patients, and they were followed for 9 to 24 months (mean 16 +/- 5 months). Twelve patients in group A had complete resolution of the thrombus and three had a significant decrease in the size of the thrombus (greater than or equal to 50% of initial thickness) during the first trimester after acute infarction. In group B the thrombus resolved in nine patients and was significantly diminished in four during the first trimester of follow-up. In group C the thrombus resolved in two patients during the first trimester and showed a significant decrease in size in two patients during the second trimester of follow-up. Two patients in group C initially had recurrent transient cerebral ischemic attacks, which did not recur after aneurysmectomy. One patient in group C had a peripheral embolic episode in the femoral artery.(ABSTRACT TRUNCATED AT 250 WORDS)