The incidence of left ventricular thrombosis after acute transmural myocardial infarction has been evaluated with two-dimensional echocardiography. To assess the preventive action of early anticoagulation with full-dose heparin, 90 patients, admitted within 5.2 +/- 4.6 hours after the onset of symptoms of their first episode of acute myocardial infarction (46 anterior and 44 inferior), were prospectively studied. Patients were randomly assigned either to therapeutic anticoagulation with heparin or to no anticoagulant therapy. Serial two-dimensional echocardiograms were recorded on the day of admission, the next day, days 4 to 7 and days 20 to 50 to detect left ventricular thrombus and to assess global left ventricular performance. On the first echocardiogram (10.3 +/- 8.0 hours after the onset of symptoms) no thrombus was visualized. In 44 patients with inferior myocardial infarction (23 receiving heparin and 21 not receiving heparin) no further left ventricular thrombus developed. In 46 patients with anterior myocardial infarction, 21 additional thrombi developed (45.6%) within 4.3 +/- 3.0 days after the acute event. Thrombus developed in 8 (38%) of 21 patients receiving heparin, compared with 13 (52%) of 25 patients not receiving heparin. This difference in ventricular thrombosis was not statistically significant (chi-square with the Yates correction = 0.76; NS). No difference was found between the subgroups in terms of clinical variables, infarct size, hemodynamic impairment, intensity of the inflammatory process and quantitative two-dimensional echocardiographic and cineangiographic left ventricular function. It is concluded that early anticoagulation with heparin reduced by 27% the incidence of left ventricular thrombus formation in anterior acute transmural myocardial infarction, and this relative risk reduction was not statistically significant when compared with findings in the untreated group.