One hundred forty-two deaths among 743 men ages 50 - 65 years who had been examined and followed 5 - 10 years were investigated and classified on the basis of clinical information from medical and non-medical observers, ECGs and autopsies. A classification based on the condition of the circulation immediately before death appears to be most relevant to studies of sudden death. In 58% of the cases, the subjects collapsed abruptly and his pulse ceased without prior circulatory collapse (arrhythmic death); in 42%, the pulse ceased only after the peripheral circulation had collapsed (deaths in circulatory failure). Thirty-three percent of arrhythmic deaths and 10% of deaths in circulatory failure occurred in a setting of clinical evidence of acute ischemic heart disease (p less than 0.005). Forty-five percent of arrhythmic deaths were preceded by chronic congestive heart failure without circulatory collapse. Ninety-three percent of final illnesses that lasted less than 1 hour ended in arrhythmic deaths; 74% lasted more than 1 day ended in deaths in circulatory failure (p less than 0.001). Eighty-eight percent of deaths that occurred outside of the hospital were arrhythmic; 71% of deaths that occurred in the hospital were deaths in circulatory failure (p less than 0.001). Ninety percent of deaths in which the primary cause of the final illness was heart disease were arrhythmic; 86% of deaths in which the primary cause was other than heart disease were deaths in circulatory failure (p less than 0.001). Ninety-one percent of deaths precipitated by an acute cardiac event were arrhythmic; 98% precipitated by acute respiratory obstruction, hemorrhage, infection, stroke or other noncardiac events were deaths in circulatory failure (p less than 0.001).