In order to determine the responsibility of coronary artery disease in unexpected sudden death (without known or treated cardiac disease), the authors studied 1,000 cases of "natural" unexpected sudden death in subjects under 65 years old in whom the autopsies were performed in Medico-Legal Institutes and in whom all non-cardiac causes had been excluded. Macro- and microscopic examination of the heart detected 848 potentially lethal lesions (152 autopsies failed to show the cause of death) of which 407 cases corresponded to coronary disease (atherosclerosis in 340 and other disease in 67 cases). The atherosclerotic lesions could be classified in 3 anatomical groups: 75 isolated recent coronary thromboses; 75 recent coronary thromboses associated with multivessel stenoses; 190 coronary stenoses (> 75%) without thrombosis (139 triple vessel, 31 double vessel and 20 single vessel diseases). The presence of a recent myocardial infarct, the cardiac mass and the circumstances of sudden death were determined in all cases with respect to the three pre-defined anatomical groups. This study, though confirming the high prevalence of coronary artery disease, evaluates it at a lower level than previous reports in the literature which takes into consideration "unexpected sudden death" of known and treated coronary patients, and also has allowed identification of other potentially lethal lesions (such as structural abnormalities of the His-Purkinje system) which are undetectable even with sophisticated paraclinical investigations.