The Physicians' Health Study was a randomized, double-blind placebo-controlled primary prevention trial designed to test whether 325 mg aspirin every other day reduced risks of cardiovascular disease and whether 50 mg beta-carotene on alternate days decreased cancer incidence among 22,071 male U.S. physicians, aged 40-84 years in 1982. Compliance with study pills, the use of non-study aspirin and platelet active drugs, specific side effects of aspirin, the incidence of conditions indicating aspirin use, and study outcomes were assessed at six month intervals during the first year and annually thereafter. The blinded aspirin component of the trial was terminated early and participants were unblinded on January 25, 1988, due to the emergence of a statistically extreme benefit of aspirin on both fatal and nonfatal myocardial infarction, as well as the extraordinarily low cardiovascular mortality rates among study participants.