The Framingham Heart Study, initiated in 1948, was designed as a longitudinal investigation of constitutional and environmental factors influencing the development of cardiovascular disease in men and women free of these conditions at the outset. The first person was examined in September 1948 and four years later 5,209 persons had received their first examination. The group has now been followed in the Study for twenty-four subsequent biennial examinations. The need for more information arises from the fact that the epidemiology of cardiovascular disease is incompletely known, estimation of risk is imprecise, and the pattern of cardiovascular disease incidence is changing; these changes need to be documented. Changing patterns of cigarette smoking, nutritional habits and exercise patterns as well as changes in other aspects of lifestyle and advances in medical care may all influence future morbidity and mortality rates for cardiovascular disease. New cases of clinically recognizable cardiovascular disease are less predictable in disease free subjects over age 60, and an already large and growing proportion of the total cardiovascular disease events is now first expressed at these ages. Recently available, technologically advanced measurement tools such as echocardiography need to be evaluated as risk assessors in various age groups. Similarly, the predictive efficacy of recently available lipoprotein cholesterol and apoprotein measurements need to be demonstrated for early onset (age less than 60) as well as later onset cardiovascular disease. Thus, as changes in early detection and treatment of cardiovascular disease advance, prospective epidemiology is needed to document the value and impact of these changes in an organized fashion. Finally, the availability of prospective data on two generations adds to the uniqueness of the Framingham Study among ongoing studies of CVD epidemiology. A determination of the extent and mechanisms by which cardiovascular diseases cluster in families is another important goal of the Framingham Study.