Posterior urethral valves are the most common cause of lower urinary tract obstruction. Only males are affected. The valves are in fact hypertrophic mucosal folds in th prostatic urethra. The severity of obstruction varies. The most serious forms manifest prenatally as hydronephrosis and oligohydramnion. Less serious forms are diagnosed after the birth, the symptoms then include poor urine stream, recurrent urinary tract infections or renal failure.