Vesicoureteral reflux was studied by retrograde cystograms in 75 patients with recurrent transitional cell carcinoma of the bladder. Unilateral reflux was detected in 6 of the 14 patients (42.8 percent) treated by transurethral resection of the bladder tumors. Five of these cases of reflux were anticipated and 1 was unanticipated. Among 61 patients treated by tumor resection and adjuvant instillations of thio-tepa reflux was found in 33 ureters of 25 patients (41.0 per cent). Of the 33 cases of reflux 15 were anticipated, while 18 were unanticipated. All the anticipated cases of reflux resulted from the direct effect of tumor resection on the ureteral orifice. The effect of thio tepa on bladder mucosa and the ureteral orifices is suggested as a possible cause of the unanticipated cases of reflux.