During repair of 12 atherosclerotic abdominal aortic aneurysms, fresh samples of aneurysm wall were obtained. Histology confirmed the presence of advanced atherosclerosis associated with medial thinning and a variable aortic adventitial chronic inflammatory cell infiltrate. Monoclonal antibodies were used to identify the inflammatory cells throughout the aortic wall. The majority of lymphocytes in the aortic adventitia were B‐cells. B‐cells were not present in atheromatous plaques. T‐cells, predominantly T‐helper cells, were found in atheromatous plaques and in aortic adventitia. The majority of lymphocytes and macrophages in aortic adventitia and most vascular endothelial cells were HLA‐DR positive. Ki‐67 staining was found in B‐cells and T‐helper cells, indicating that these cells were proliferating. Occasional lymphocytes were BerH2 positive, indicating that some lymphocytes were activated. These findings suggest that chronic periaortitis is an active, immunoiogically mediated, local complication of advanced human atherosclerosis.