Fifty patients for whom an ileal replacement of the ureter was indicated were prospectively randomized among 2 treatment groups according to the surgical technique used. In group 1 the standard operation was performed. In group 2 a tailored and valved (modified) ileal ureter was used. Patients were followed every 3 to 6 months for 12 to 28 months. Kidney functions were evaluated by 99m technetium diethylenetriaminepentaacetic acid scans and by determination of the 24-hour creatinine clearance. Urine cultures were obtained, and urine viscosity and flow rates were measured. A urographic study, including ascending cystography, was done every 6 months. Results indicated that the modified operation is functionally superior to the standard procedure. It provides efficient unidirectional flow of urine from the kidney to the bladder with less mucus secretion. This was ultimately reflected as better renal function when it was quantitated by renography and/or chemical clearance.