This study was designed to assess, by two-dimensional echocardiography, the effects of anticoagulant therapy on left ventricular thrombosis detected after acute myocardial infarction. Thirty-eight patients with left ventricular thrombi detected by two-dimensional echocardiology within 5 weeks (mean 4) of the onset of infarction were randomly assigned to the following groups: group A consisted of 19 patients who received oral anticoagulants (acenocoumarin 1-6 mg daily regulated to keep prothrombin time within the range of 25 to 35%) and group B which consisted of 19 non-treated control patients. Seventeen patients from both groups were restudied 15 days, 3 months and one year later to evaluate the changes in size of thrombi. Echocardiographic examinations were read blindly; a significant decrease in ventricular thrombus size was taken as a greater than or equal to 5 mm reduction of thickness in the apical views. In Group A, 9 patients showed a complete resolution of thrombus at the 15 day study; at one year, thrombus had resolved in 15 and persisted unchanged in size in 2 patients. The mean dimension of thrombi in patients of group A was 18 +/- 6.6 mm at the screening examination and decreased to 6.6 mm, 3.8 mm and 2.2 mm, respectively, at 15 days, 3 months and one year follow-up studies. Among 17 patients of group B at the 15 day study, two had resolution of thrombus and 15 were unchanged; at the one year examination thrombus was resolved in 4, decreased in size in 4 and persisted unchanged in 9 patients. Analysis of variance of the dimensional changes of thrombi in the two groups of patients confirmed a significant efficacy of anticoagulant therapy (P less than 0.001). On the basis of our results we conclude that full-dose anticoagulant therapy, started early (within 5 weeks) after acute myocardial infarction, is effective in the resolution of left ventricular thrombosis.