Ten cases of post-inflammatory aneurysm of the thoracic aorta were studied morphologically using aortic wall and aortic valve tissue resected during the surgical repair of the lesions. Four cases of aortic aneurysm in Behcet's disease showed massive perivascular infiltration of the lymphocytes and plasma cells in adventitia and vasa vasorum. Fibrous obliteration of the vasa vasorum was seen, and neutrophilic infiltration was also associated in two cases with recurrent prosthetic valve failure. Three cases of aortic aneurysm associated with Takayasu's arteritis showed similar histologic features, but giant cells and histiocytic reactions were additional findings. Plasma cell infiltration was more pronounced compared to the Behcet's aortitis. One case of luetic aortitis showed a non-specific chronic inflammatory reaction with florid endothelial cell proliferation. One case of ankylosing spondylitis showed fibrous thickening at the periannular portion of the aortic valve cusps, but this finding was also seen in an aortic valve of a patient with Behcet's disease. One case without any disease association showed similar features of chronic inflammation with granuloma formation. The findings suggest that they have similar histologic features indistinguishable in terms of the clinical disease association, but a granulomatous reaction could be expected in Takayasu's arteritis.