During the past decade some urologists and radiologists have doubted the existence of posterior urethral valves. It has been suggested that the primary lesion in patients thought to have urethral valves is bladder neck obstruction. During the past seven years we have seen an average of three new cases of posterior urethral valves per annum. In all cases the valves themselves, not the bladder neck, obstructed the urethra. The urologist may miss valves at urethroscopy, but use of the new fibreoptic endoscope has improved his chances of seeing them. Posterior urethral valves may not be demonstrated radiologically if the radiologist does not use a rapid recording device or if the patient does not micturate forcefully during micturating cystourethrography. Failure to detect the valves may lead to an erroneous diagnosis of bladder neck obstruction.