A proportion of abdominal aortic aneurysms are found at laparotomy to have thick white walls and extensive peri-aneurysmal adhesions. Graft replacement in these patients is often difficult and associated with an increase in morbidity and mortality. In this study 103 patients with a clinical diagnosis of aortic aneurysm were scanned with computerized tomography to determine whether this condition could be diagnosed preoperatively. Of these, 15 showed diagnostic radiological criteria of peri-aortic fibrosis. Laparotomy revealed no false negatives and only one false positive. Clinical, serological and pathological findings have been compared to those with 'normal' aneurysms. In view of the similarities between this condition and retroperitoneal fibrosis, 5 patients were treated with Prednisone 5 mg twice daily for up to 18 months and a CAT scan technique developed to monitor the degree of thickening quantitatively. Abdominal pain disappeared rapidly, and sequential scanning showed a decrease in peri-aortic fibrosis. Measurements showed the fibrosis only to disappear, with no increase in the diameter of the aortic lumen. In some cases, the lumen was only minimally dilated, although clinical examination suggested an aneurysm. Consequently we suggest that the term peri-aortic fibrosis is a better description of this condition whose aetiology is still obscure, and in which aneurysm formation, when present, may simply be consequent upon the fibrotic reaction in the wall.