A review of the histology of atherosclerotic aneurysms shows that all are accompanied by adventitial inflammation and fibrosis, usually slight. Severe degrees of this periaortic inflammation are visible macroscopically and give rise to the conditions variously known as inflammatory aneurysm or perianeurysmal retroperitoneal fibrosis. The disease appears identical to most cases of idiopathic retroperitoneal fibrosis, except that in this condition the aorta is not dilated. Less commonly similar adventitial inflammation and fibrosis are seen around coronary arteries. The appearances suggest that all these forms of inflammation are essentially identical and probably due to hypersensitivity to antigens in the atheromatous plaque.