Six cases of dissecting aneurysm of the aorta complicating aortic valvular disease in which aortic stenosis was dominant are described. In 3 of the cases the aortic complication occurred after the natural aortic valve had been replaced by a prosthetic valve. The longest postoperative period was 4 months. The factor common to all cases in this series was cystic medial necrosis of the aorta. The authors agree with McKusick that aortic dissecting aneurysms complicating aortic valvular disease are more than a chance association and are related to cystic medial necrosis of the aorta. The latter, in turn, is a consequence of the altered hemodynamics of aortic valvular disease, particularly stenosis.