The Coronary Drug Project Aspirin Study (CDPA), started in late 1972 and terminated in early 1974, involved men selected from three groups originally receiving dextrothyroxine or estrogen therapy in the Coronary Drug Project. All patients had a history of myocardial infarction, most of them several years earlier. Length of follow-up ranged from 10-28 months (average 22 months). A total of 1529 patients were recruited and were randomly assigned on a double-blind basis to aspirin therapy -- one 324-mg tablet three times daily or a corresponding placebo treatment. Data indicate a high level of adherence to the study protocol. No major differences were recorded in use of nonstudy medications in the two treatment groups. Overall mortality was 5.8% in the aspirin group and 8.3% in the placebo group (an observed difference of 30%). This difference, suggestive of a beneficial effect for aspirin in the treatment of post-myocardial infarction men, was not large enough to be conclusive.