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 the risk of cardiovascular disease and whether 50 mg beta-carotene on alternate days decreased cancer incidence among 22,071 male physicians, aged 40-84 years in 1982. On December 18, 1987, the external Data Monitoring Board recommended early termination of the blinded aspirin component of the trial, based on 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.