Ureteral replacement by ileum is an accepted technique in a highly selective patient population. Two major contraindications in using an ileal ureter are compromised renal function (serum creatinine greater than 2) and a functionally abnormal bladder. We used ileum to bridge a ureteral defect in animals with half of a solitary kidney and low grade azotemia to see if the antirefluxing mechanism of the lower ureter prevented further deterioration in renal function. Twelve female mongrel dogs underwent a right nephrectomy, followed by a left partial nephrectomy six weeks later. Group I (six dogs) had a six cm. segment of ileum interposed between the upper and lower ureteral segments (nonrefluxing). Group II (five dogs) had a ten cm. segment of ileum placed from the upper third of the ureter to the bladder (refluxing). Cystograms, intravenous pyelograms, serum electrolytes, BUN and creatinine were obtained preoperatively, six weeks after the right nephrectomy, one month after left partial nephrectomy and six months after ileal replacement prior to sacrifice. The BUN and creatinine deteriorated in Group II compared to Group I, p = .02 and p = 0.4 respectively (Mann-Whitney test). The BUN and creatinine also deteriorated between one month after left partial nephrectomy and six months after ileal replacement within Group II, p = .07 and p = .14, respectively (Wilcoxon matched-pairs test) but not in Group I. These data suggest that the antirefluxing mechanism of the lower ureter might prevent further deterioration in renal function. We feel that ileum can be used with caution, as an interposition in compromised renal function.