Advances in molecular genetics, genetic epidemiology, population genetics, and identification of new risk factors and clusters of risk factors for cardiovascular disease made 1991 an opportune time to take advantage of the extensive information about cardiovascular disease, pre-clinical atherosclerosis, and risk factors in individuals who had been examined in ongoing, state-of-the-art epidemiological studies supported by the NHLBI. By recruiting first degree relatives of random samples of such defined populations, FHS obtained information about familial aggregation, genetic and environmental contributions to variance in continuous variables, and the frequency and distribution of elevated levels of risk factors and of selected major genes in the general population.