Pelvic exenteration with diversion of both ureters was performed in 40 patients with advanced pelvic cancer and one ureter occluded on preoperative intravenous pyelograms (IVP). In 29 instances, renal function returned. Preoperative periods of occlusion were not always determinable; where they were, the average occlusion time followed by functional return was three to four months. Among those that did not show functional return, the average estimated occlusion time was 7 1/2 months. In 11 patients who had preoperative IVP over varying periods, it was possible to fix rather accurately dates of complete ureteral occlusion. There was return of function in seven patients with periods of occlusion varying from two to four weeks, and in one patient with 3 1/2 months occlusion. In two patients with occlusion. for 165 days, impaired function returned. In one patient with 100 days occlusion, function returned slowly and remained impaired.