The histologic changes that occur in the media of the normal aorta at various ages were studied in 100 normal aortas. These changes encompassed (1) cystic medial necrosis, defined as pooling of mucoid material; (2) elastin fragmentation, characterized by disruption of elastin lamellae; (3) fibrosis, defined as an increase in collagen at the expense of smooth muscle cells; and (4) medionecrosis, defined as areas with apparent loss of nuclei. The changes showed a striking correlation with age and may represent the normal aging process for the aorta as determined histologically. The alterations showed a close relation in onset and location within the media, suggesting a phenomenon of injury and repair caused by hemodynamic events. These findings in the normal aging aorta reveal that none of the histologic changes observed can be regarded as the specific structural alteration responsible for the development of dissecting aneurysm.