This trial was part of the Physicians Health Study, an ongoing, randomized, placebo-controlled clinical trial of aspirin in the prevention of cardiovascular mortality and of beta-carotene in the prevention of cancer. Following randomization, each of the 22,071 physicians enrolled was assigned to one of four groups to take either aspirin or its placebo and beta-carotene or its placebo. Follow-up questionnaires were sent 6 and 12 months after randomization and every 12 months thereafter. The randomized aspirin component of the trial was terminated early (January 1988), after an average followup of approximately 5 years, because of a statistically extreme 44 percent reduced risk of a first myocardial infarction in the aspirin group.