In previous investigation of the normal aging aorta, the claimed specificity of alterations in the media in the pathogenesis of dissecting aneurysm of the aorta was challenged. The concept was promoted that these changes are nonspecific and caused by general hemodynamic events within the aorta. In this investigation the aortic media was studied in patients with a dilated ascending aorta, whose hemodynamic profile is known to be altered. The results were compared with data obtained from the study of aortas with complete or incomplete dissection and aortas from patients with Marfan's syndrome, a condition known to predispose to dissection. Only quantitative differences were found between the normal "aging" aorta and the overtly abnormal aorta. The pathogenesis of dissected aneurysm, therefore, is considered to be initiated by processes if injury and repair within the aortic wall, consequent to hemodynamic forces. The histologic features of the media previously implicated as the specific underlying defect appear to represent the morphologic substrate of this traumatizing and reparative process. This process may gradually lead to dilatation of the aorta and, according to Laplace's law, a vicious cycle may ensue that may lead to further complications. Local circumstances determine whether a dilated aorta will rupture or whether an incomplete or complete dissection will occur. The fact that the aorta in patients with Marfan's syndrome shows basically the same structural alterations supports the concept proposed. The underlying connective tissue disorder in these patients will lead to complications at an earlier age. Dissecting aneurysm therefore is part of a spectrum of lesions that have as a common denominator the process of injury and repair.