OBJECTIVE In voiding cystourethrography, the urethral catheter may or may not be left in place during voiding. The main argument for removing the catheter is that the diagnosis of posterior urethral valves may be missed because the catheter can hold open the valve, efface it, and render it invisible. However, if the catheter does not prevent the diagnosis of urethral disease, it is preferable to leave it in place. The catheter makes it possible to repeat the procedure easily if necessary, and using it to drain the bladder provides information about ureteric obstruction in the presence of vesicoureteric reflux. Accordingly, the purpose of this study was to determine whether leaving the urethral catheter in place throughout voiding cystourethrography affects the efficacy of the procedure for the diagnosis of posterior urethral valves. METHODS Three radiologists reviewed the preoperative voiding cystourethrograms obtained in 48 boys who ranged in age from 1 day to 10 years old (mean, 1.5 years). All patients had a diagnosis of posterior urethral valves made at cystoscopy, which was used as the gold standard. The voiding cystourethrogram was obtained with a catheter in place during voiding in 28 (58%) of the 48 boys, without a catheter in 17 (35%), and with and then without a urethral catheter during the voiding phase of the study in three (6%). RESULTS Posterior urethral valves were detected on 25 (89%) of the 28 voiding cystourethrograms obtained with a urethral catheter in place and in 15 (88%) of the 17 voiding cystourethrograms done without a urethral catheter. The five children in whom posterior urethral valves had been diagnosed by cystoscopy but were not detected on voiding cystourethrography had no dilatation of the posterior urethra nor any other evidence of obstruction; these were possibly false-positive cystoscopic diagnoses. CONCLUSIONS Our results show that a urethral catheter does not obscure posterior urethral valves in boys and need not be removed routinely during the voiding phase of voiding cystourethrography.