A combined pathological and radiological study of descending aortae from 15 subjects from 2 to 80 yrs of age was conducted to investigate changes in the segmental vessels with advancing aortic atherosclerosis. There were (i) small hillocks of musculo-elastic intimal thickening adjacent to the apex (centre of the flow divider) in the ostia of the segmental vessels in young subjects, (ii) progressive intimal thickening, distortion and narrowing of the ostia of the segmental vessels (iii) obliteration or loss of some ostia in advanced atherosclerosis of the distal abdominal aorta and (iv) gross discrepancy in severity of atherosclerosis between the aorta and its branches, the intimal proliferation tapering rapidly in the branches and often being minimal in the outer part of their intramural courses. Distal ramifications exhibited occlusions which radiographically were probably embolic or possibly artifactual. Atherosclerosis was rarely of significant degree histologically within the proximal 2 cm of the segmental branches.