Among 119 cases of fatal dissecting aneurysm of the aorta, exclusive of those iatrogenically caused or associated with arachnodactyly or aortic stenosis, there were observed 11 cases of congenital bicuspid aortic valve (9%). The ages ranged from 17 to 69 years, five of the patients being 29 years old or younger. Among the latter, three had coarctation of the aorta and one had Turner's syndrome without coarctation. In one of the older patients, aortic insufficiency was present. Hypertension was either established or inferred from cardiac weight in 73% of the cases. In each case, cystic medial necrosis of the aorta was present. Prolapse of valves other than the aortic was observed in 45% of the cases with bicuspid aortic valve. Compared to an estimated incidence of bicuspid aortic valve of about 1 to 2% in the population, the high incidence among subjects with dissecting aneurysm suggests a causative relationship between bicuspid aortic valve and aortic dissecting aneurysm.