Three hundred patients with acute myocardial infarction (AMI) were studied. Half of them received anticoagulant treatment with heparin and acenocoumarin (Group I) The other 150 patients did not have anticoagulant therapy (Group II) Both groups were similar in age, sex, prior history of diabetes, arterial hypertension or smoking habits. The incidence of persistent angina, arrhythmias, pulmonary embolism and re-infarction was not different for both groups. The mortality was slightly higher in group II. Of the patients who died, the autopsy demonstrated fresh thrombi in all cases of both groups; in addition, most of the patients of group II hemorrhage of the coronary artery wall was found.