In an epidemiologic study of the total population of Tecumseh, Michigan, 98 deaths from coronary heart disease were observed between 1959 and 1965. Forty-five of the fatalities occurred within 1 hour of the onset of symptoms and were classified as sudden. The proportion of sudden deaths among men was nearly twice that among women, and the incidence increased progressively with age in both sexes.Hypertensive heart disease, coronary heart disease, or diabetes mellitus had been detected on prior examination in 62% of those who died suddenly. Physiologic abnormalities associated with a high risk of coronary heart disease were also found more frequently than in the total Tecumseh population.All but seven of the persons who died suddenly had abnormalities including arrhythmias and conduction defects which were detected in the standard 12-lead electrocardiogram.Although sudden deaths often seem to occur without warning, the victims are predisposed by conditions which are detectable long before the catastrophic event. The identification of conditions which are precursors of sudden death from coronary heart disease permits a rational consideration of possible preventive measures.