Of 141 patients with abdominal aortic aneurysms diagnosed at the Mayo Clinic from 1950 through 1959 who were not operated on, follow-up information was obtained from 137 patients (97.1 per cent). Of those followed 1 year or more, 87.6 per cent survived 1 year or more; 52.5 per cent survived 3 years, and 36.4 per cent survived 5 years after diagnosis. These results indicate that the prognosis of abdominal aortic aneurysm in this selected group of patients was somewhat better than that previously reported. The prognosis and survival of patients who had a history of associated cardiovascular disease at the time of diagnosis of aneurysm is distinctly less good than those of patients who had no such history. Of those with such a history, 75.0 per cent survived 1 year, 33.3 per cent survived 3 years, and 20.0 per cent survived 5 years. Of those without such a history 95.7 per cent survived 1 year, 63.2 per cent survived 3 years, and 50.0 per cent survived 5 years. Patients with abdominal aortic aneurysms and associated cardiovascular diseases are more likely to die of cardiovascular complications other than ruptured aneurysms. Patients with abdominal aortic aneurysms who have no evidence of associated cardiovascular disease are more likely to die from a ruptured aneurysm than from anything else. It would appear that small asymptomatic abdominal aortic aneurysms in patients with associated cardiovascular disease may be carefully observed until signs of expansion of the aneurysm or symptoms from the aneurysm appear.