The ARIC study is a large population-based, prospective, multicenter study supported by NHLBI which began in 1986, and is designed to investigate the etiology and natural history of atherosclerosis and cardiovascular risk factors. It involves investigation of four cohorts, each with 4,000 adults ages 45 to 65 from four different communities. The study conducts surveillance for hospitalized myocardial infarction, fatal cardiovascular disease, and stroke in its cohorts and of the community from which they are drawn. Events are classified by a Morbidity and Mortality Classification Committee using standardized criteria.