Anatomoclinical and chronobiological aspects of sudden death (SD) of 671 subjects observed at the Emergency Room of Ferrara Hospital from January 1983 to December 1991 were prospectively investigated. 3 groups stratified by age were considered: group A with age < 65 years (no. = 251, 37.4%), group B with age between 65 and 74 years (no. = 210, 31.3%), and group C with age > or = 75 years (no. = 210, 31.3%). SD was classified on the basis of either anatomopathological (i.e.: acute myocardial infarction, acute myocardial failure, intracerebral hemorrhage, rupture of aortic aneurysm, pulmonary embolism), and clinical findings (i.e.: arrhythmic death and circulatory failure death). Patients were grouped into six 4-hour periods according to the time of onset of symptoms, and circadian distribution was tested for uniformity by a chi 2 test for goodness of fit. The analysis found in group C an increased frequency of SD due to pulmonary embolism and rupture of aortic aneurysm, and an increment of males/females ratio in deaths from cardiac causes. In group C the mean age of SD, in particular from acute myocardial infarction, pulmonary embolism or arrhythmic death proved higher in females compared to males. Only for group B was a significant circadian periodicity found for SD from acute myocardial infarction and arrhythmic death, with a peak in the morning.