A total of 50 patients for whom an ileal ureter was indicated was prospectively randomized between 2 treatment groups according to the surgical technique used. In group 1 the standard ileal ureter procedure was performed, while in group 2 a tailored and valved (modified) ileal ureter was fashioned. Of the patients 19 in group 1 and 23 in group 2 were available for followup of 69 +/- 5.8 months (range 60 to 80). Among the 23 patients in group 2, 5 (22%) experienced reflux due to dessusception of the nipple valve and 4 (17%) had bladder stones. Metabolic acidosis was observed in 9 patients in group 1 (47%) and 4 in group 2 (17%). Urinary outflow obstruction occurred in 5 patients in group 1 and 1 in group 2. Although renal function was comparable among patients in both treatment groups, comparison between patients in group 1 and those without reflux in group 2 showed better renal function in the latter group. We conclude that reflux prevention is of central importance in preservation of renal function in patients with an ileal ureter. A distal nipple valve at the ileovesical anastomosis is associated with significant problems. These observations would urge urologists to search for a more suitable procedure that provides an efficient antireflux principle with minimal adverse sequelae.