Epidemiologic studies demonstrating increased morning incidence of sudden cardiac death. To determine if sudden cardiac death shows a circadian variation, the mortality records of the Massachusetts population and the Framingham Heart Study were analyzed. The Massachusetts mortality data analysis revealed an increased morning incidence for out-of-hospital cardiac deaths similar to that previously described for nonfatal myocardial infarction. The records of the Framingham Heart Study allowed more comprehensive analysis of the time of sudden cardiac death. In this study, a prominent circadian variation was also identified in which frequency of sudden cardiac death increased sharply between 6 A.M. and 9 P.M.; distribution was fairly even throughout the rest of the day. This circadian pattern may be explained in part by physiologic changes that increase the likelihood of ventricular fibrillation or increase the risk of thrombosis in the morning hours. The possible role of the morning increase in physical and mental activity as a trigger of sudden cardiac death requires further investigation. 