Aortoiliac aneurysm is a rare cause of ureteral obstruction. We report a case of perianeurysmal fibrosis(PAF) associated with aortoiliac aneurysm resulting in hydronephrosis. CT and MRI findigns of PAF are presented. In patient with hydronephrosis of unknown cause, PAF should be included among the differential diagnosis. The diagnosis of PAF is easily made with CT and MRI findings.