The optimal diameter at which replacement of the ascending aorta should be performed in patients with bicuspid aortic valve disease is not known. We reviewed all patients with bicuspid aortic valves undergoing aortic valve replacement at our institution from 1979 through 1993 (n = 201). Patients undergoing concomitant replacement of the ascending aorta were excluded. Follow-up was obtained on 98% of patients and was 10.3 +/- 3.8 (mean +/- SD) years. The average patient age was 56 +/- 15 years, and 76% were male. The ascending aorta was normal (<4.0 cm) in 115 (57%) patients, mildly dilated (4.0-4.4 cm) in 64 (32%) patients, and moderately dilated (4.5-4.9 cm) in 22 (11%) patients. All patients with bicuspid aortic valves with marked dilation (>5.0 cm) underwent replacement of the ascending aorta and were therefore excluded. Fifteen-year survival was 67%. During follow-up, 44 patients required reoperation, predominantly for aortic valve prosthesis failure. Twenty-two patients had long-term complications related to the ascending aorta: 18 required an operative procedure to replace the ascending aorta (for aortic aneurysm), 1 had aortic dissection, and 3 experienced sudden cardiac death. Fifteen-year freedom from ascending aorta-related complications was 86%, 81%, and 43% in patients with an aortic diameter of less than 4.0 cm, 4.0 to 4.4 cm, and 4.5 to 4.9 cm, respectively ( P < .001). Patients undergoing operations for bicuspid aortic valve disease should be considered for concomitant replacement of the ascending aorta if the diameter is 4.5 cm or greater.