We studied the histologic and histometric features of the aortic media in cases of dissecting aneurysm in order to clarify the pathologic background of this condition. The results were then compared with cases of true aneurysm and control specimens of "normal" aging aorta. Most cases of dissecting aneurysm and true aneurysm, as well as a number of control specimens, showed severe cystic medial necrosis, and therefore this feature does not appear to be specific to dissecting aneurysm. Fibrosis was noted in the process of repair of medionecrosis in dissecting aneurysm, true aneurysm, and controls. The main pathologic features of the aortic media in dissecting aneurysm included a higher grade of elastin fragmentation (offensive factor) and less fibrosis (defensive factor). Our findings indicate that the pathologic balance between these offensive and defensive factors is an important consideration when evaluating the pathogenesis of dissecting aneurysm.