Nine operatively treated patients with abdominal aortic aneurysm and associated perianeurysmal fibrosis are reported. The fibrotic reaction closely corresponded to the extent of the aneurysmal disease. In four patients only the periaortic tissue was involved, whereas in the other five there was also involvement of the tissue surrounding the inferior vena cava, the duodenum and one or both ureters. A tentative preoperative diagnosis of perianeurysmal fibrosis was made in three patients and in one of them this was confirmed at computer tomography. In view of the similarity in clinical and microscopic appearance it is suggested that "perianeurysmal fibrosis", "inflammatory aneurysms" and retroperitoneal fibrosis" associated with abdominal aortic aneurysms might represent the same disease or different stages of it.