The aneurysms in 35 380 autopsies over a period of 25 years have been investigated with reference to etiology, sex, age, morphology, localization, additional factors, complications and clinical diagnosis. There were a total of 1706 aneurysms (362 of cerebral arteries, and 1344 of the aorta and further arteries). The most frequent current cause of aneurysm is arteriosclerosis (80%), followed by idiopathic medianecrosis of Erdheim-Gsell (9.7%) and syphilis (3.7%). Arteriosclerotic and syphilitic aneurysms are most frequently found in patients over 60. In medianecrosis the age ceiling is 40-70 years for men and over 60 years for women. With the exception of medianecrosis (dissecting aneurysms), the great majority of cases are morphologically true aneurysms. The most frequent localization of rupture is the retroperitoneal tissue (arteriosclerosis), the pericardial sac and the pleural cavity (syphilis, medianecrosis). Approximately every fourth aneurysm found in the autopsy series had been clinically diagnosed.