When the Physicians Health Study was established in the early 1980s by the NHLBI to collect data on a stable population at risk for heart attack, only male physicians were studied because males were thought to be at higher risk than females for ischemic heart problems. The study population consists of 22,071 male U.S. physicians who were aged 40 to 84 years in 1982. The subjects have no history of myocardial infarction, cancer, kidney disease, renal disease, or any other contraindication to the use of aspirin or beta-carotene, including regular use of corticosteroids.