Although neither aspirin nor oral anticoagulants have been conclusively shown to reduce mortality in patients surviving myocardial infarction, both have been widely used for that purpose. In the present clinical trial we compared the effects of aspirin (0.5 g given three times a day) and oral-anticoagulant therapy. Of 6908 patients considered for entry, 1303 were randomized to anticoagulant (652) or aspirin (651) an average of 11.4 days after the onset of myocardial infarction and were followed for 6 to 59 months (mean, 29 months). There were 65 deaths in the anticoagulant group and 72 in the aspirin group. The number of patients with reinfarctions was higher in the aspirin group (33 vs. 20). None of these differences were statistically significant. Almost twice as many patients were withdrawn from therapy in the aspirin group. There were 54 per cent more patients with gastrointestinal events in the aspirin group and four times more patients with episodes of severe bleeding in the anticoagulant group. We conclude that aspirin in the dosage used in probably not different from oral anticoagulants in affecting mortality and morbidity after a myocardial infarction. However, this study does not consider the effectiveness of either agent in comparison to no antithrombotic therapy -- an issue that remains unsettled.