In 63 resected thoracic aortic aneurysms, the commonest histological finding (45 cases, 71.4%) was cystic medionecrosis. These cases formed two groups, 29 with widespread fragmentation and loss of elastic tissue (elastopathy) and 16 cases without elastopathy who were older and included most of the 18 cases of dissecting aneurysms. Thirteen patients had the Marfan syndrome, Io showing cystic medionecrosis with elastopathy, indistinguishable form the cases with no Marfan stigmata although partial ‘dissections’ were mainly found in the Marfan patients, Histological appearances ranged from normal to complete loss of media. Cystic changes in muscle fibres apparently preceded elastic fragmentation. Fourteen cases (22%) had aortitis: 4 were syphilitic and 3 of other known aetiology. In 7 patients the aetiology of the inflammatory process was unknown and appearances included granulomatous infarct-like lesions and necrotizing aortitis or changes indistinguishable histologically from syphilis.