The records of 761 patients with acute myocardial infarction were reviewed to assess the value of optimum anticoagulant therapy and specifically to ascertain the relationship between levels of prothrombin control and fatalities. Results showed that oral anticoagulant therapy was associated with a significantly lower fatality rate than was no treatment; however, there was no relationship between levels of therapy and fatalities. Patients maintained at levels of 10% to 25% of normal prothrombin activity had the same incidence of thromboembolism and death as did those with poorer control and no advantage could be found for optimum therapy as it is now defined. Patients treated solely with heparin throughout hospitalization were considered separately. In this group the incidence of thromboembolism and death was high, resembling the rates obtained without anticoagulation and creating suspicion that heparin is not valuable in acute infarctions.