A total of 66 patients with low grade, low stage transitional cell carcinoma of the bladder who were treated with intravesical bacillus Calmette-Guerin (BCG) underwent cystourethrography to detect vesicoureteral reflux. BCG was instilled weekly for 6 weeks and monthly thereafter for up to 24 months. Whenever vesicoureteral reflux was found or morphological abnormalities were detected on excretory urography (IVP) an isotope renal scan was performed to evaluate the relative renal function. Vesicoureteral reflux was found in 13 patients (19.7%): 10 had grades 1 and 2A, and 3 had grade 2B reflux. The number of BCG instillations ranged from 8 to 22. IVPs were normal in 11 patients. In 2 patients mild unilateral dilatation was present before BCG instillations, and this remained unchanged during and after therapy. None of the 13 patients with vesicoureteral reflux had IVP features suggestive of urinary tuberculosis. In 11 patients the refluxing renal systems had normal relative renal function (50 to 55%). Two patients had a decrease to 40% of the relative renal function with normal IVPs, suggesting a nonBCG related cause. We conclude that BCG therapy is safe in patients with minimal reflux.