During a ten-year period (1959 to 1969) 80 patients underwent operation for aneurysm of the ascending aorta, 24 for acute dissection and 56 for chronic dissection or fusiform enlargement. Fifteen of the patients with acute dissecting aneurysms and 46 of those with chronic lesions had aortic regurgitation; 70% of the entire series required aortic valve replacement. The predominant pathologic lesion was cystic medial degeneration in 75% and atherosclerosis in 25% of the cases. Mortality was 25% for patients with acute dissecting aneurysms, 17% for those with chronic lesions, and 20% for the entire series. We recommend aggressive surgical therapy for acute dissecting aneurysm of the ascending aorta. In our experience this is a more serious condition than acute dissection of the descending aorta.