Making a voiding cystourethrogram prior to cystoscopy has been found to be of great value in the diagnosis of the site of obstruction of the lower urinary tract in children. Valvular obstruction of the posterior urethra was found to be more common than had been supposed. In the past 3 years 9 cases have been diagnosed. Removal of the valves by transurethral resection was more satisfactory than open operation. Attention is drawn to the importance of considering the megacystis syndrome and fibroelastosis in the differential diagnosis of lower urinary-tract obstruction.