In a retrospective study, 268 patients with all three types of abdominal aortic aneurysms (AAA) were analyzed. They had been seen at the Surgical Clinic A of the University Hospital in Zurich, Switzerland, from 1961 through 1977. The follow-up included 97% of the surgical and all nonsurgical cases. This report is based on the analysis of the 167 surgically and 16 nonsurgically treated patients with asymptomatic (I) or expanding (II) AAA. Of the 16 patients not operated on, 15 died after a mean time of 13 months after examination, 57% due to rupture and 29% because of coronary heart disease. In the operative group, the 30-day perioperative mortality was 7% for the asymptomatic AAA and 14% for the expanding. Late mortality reached 25% for type I and 37% for type II after a mean survival time of 50 months. Of all surgically treated patients, 58% could be examined 1-14 (mean 4) years after aneurysmectomy. The cumulative survival rate for the 5th year reached 73% in the surgical asymptomatic group and 48% in the expanding or 62% for both; the nonsurgical group had a survival rate of only 6%. The results of the long-term follow-up are described in detail as well as the various factors related to arteriosclerosis or to surgery with statistically significant influence on prognosis. Comparison of the late results of all three types of AAA and confrontation with the spontaneous course of the disease emphasize the importance of early indication for operation.