Retroperitoneal fibrosis is associated with Riedel's thyroiditis, in which an unexpectedly high proportion of the plasma cells have been reported to contain IgA and lambda light chains. It has been suggested that retroperitoneal fibrosis and the inflammation and fibrosis in thick-walled abdominal aortic aneurysms are caused by a hypersensitivity reaction to antigens leaked from aortic atheroma. We examined cases of retroperitoneal fibrosis and aortic aneurysms in order to quantify the types of heavy and light chains in the plasma cells. A mean of 44% of the plasma cells contained IgA and 52% contained lambda light chain. These results provide further evidence of the pathological relationship between retroperitoneal fibrosis and Riedel's thyroiditis. It is suggested that a cross-reaction between antigens in mucosal surfaces and in the thyroid or retroperitoneum may be implicated, possibly involving vessel walls. Of the total plasma cells in the wall of the aortic aneurysms 24% contained IgA and 40% lambda, although there was a significant trend towards a higher proportion of IgA with increasing thickness of the wall. No definite support for a relationship between atheroma and retroperitoneal fibrosis is provided by this study.