Total and separate renal function, renal parenchymal thickness and dilatation of the upper urinary tract were studied in 40 patients preoperatively and 24 to 67 months after urinary diversion, using 51Cr-EDTA clearance test, scintillation camera renography and urography. In ten patients a continent caecal reservoir was used for diversion. In the other patients, an ileal or a colonic conduit (15 patients with each method) was used, one ureter being implanted with an anti-reflux method and the other with direct technique. Renal function following urinary diversion showed little or no deterioration in most patients. The functional outcome was not related to the method of diversion or, in the conduit groups, to the mode of ureteral implantation. Serum creatinine tests and urography were not adequate for determining loss of renal function. Radionuclide studies proved to be valuable for assessing renal function after urinary diversion.