Inflammatory abdominal aortic aneurysms. In a series of 517 operations for abdominal aortic aneurysm from 1971 to 1988 there were 45 cases (8.7%) with an inflammatory aneurysm with a typical thick glistening whitish fibrous layer. Almost two-third of the patients had rather severe chronic or acute progressive pain in the abdomen, the back or the flank. Unilateral (7) or bilateral (2) hydronephrosis due to ureteral compression occurred in 9 patients (20%). A diagnosis of inflammatory aneurysm was made preoperatively only in 10 patients. In 8 of the 9 patients with hydronephrosis ureterolysis was done, unilaterally (6) or bilaterally (2). After ureterolysis all had complete resolution of the hydronephrosis. Preoperative diagnostic methods are excretory urography, showing medial deviation, ultrasonography and CT-scanning of the abdominal aorta. All patients with an inflammatory aneurysm should undergo aortic replacement to prevent rupture and achieve pain relief. Ureterolysis in cases of hydronephrosis is strongly recommended and may be performed safely and with excellent results. 