A clinical-epidemiological and pathological study of sudden death in a defined area of Baltimore was begun in June 1970. The background characteristics of the deaths, prodromal symptomatology and factors associated with the onset are being studied. For most of the deaths certified by the Medical Examiner a detailed postmortem examination was performed. Approximately two thirds of the arteriosclerotic heart disease (ASHD) deaths were sudden and only 26% occurred in a hospital. Sudden death patients are characterized as having a high frequency of prior heart disease, hypertension, and diabetes, recent medical care, and many prodromal symptoms. The onset is usually at home or work and rarely associated with an obvious acute precipitating event. There is considerable patient delay at onset but rarely physician delay. At postmortem examination sudden ASHD deaths have severe atherosclerosis which involves three or four vessels and is diffuse. Acute lesions are relatively rare.