Summary— During ureterolysis in a patient with “idiopathic retroperitoneal fibrosis”, fresh samples of peri-ureteric and peri-aortic tissue were obtained. An abdominal CT scan confirmed the peri-aortic distribution of the inflammation associated with advanced abdominal aortic atherosclerosis. Histology confirmed the presence of fibrosis and a variable chronic inflammatory cell infiltrate. Monoclonal antibodies were used to identify the inflammatory cells. B and T lymphocytes were present with the majority of T lymphocytes of the T helper phenotype. The majority of lymphocytes and macrophages and most vascular endothelial cells were HLA-DR positive. Ki67 and BerH2 staining was found in B cells and T helper cells, indicating that these cells were proliferating and activated. These findings compare with the characterisation of inflammatory cells associated with “inflammatory aneurysms” and with the inflammatory cells present in the spectrum of inflammation seen as a complication of advanced atherosclerosis—conditions known as “chronic peri-aortitis”. It is suggested that our findings support the view that idiopathic retroperitoneal fibrosis represents clinical chronic peri-aortitis seen in an undilated aorta.