The authors report a case of rhabdomyosarcoma of posterior urethra they observed in a 10 year-old boy who was referred for an acute urinary retention. The story of the patient included other urinary tract troubles: macroscopic haematuria and stranguria had been observed when the boy was four year old and at that time cystography and cystoscopy grave normal results. In the following years the patient presented repeatedly episodes of haematuria, stranguria, dysuria and urinary tract infections. The authors stress the slow evolution of the tumor and the difficulties for diagnosing the disease in its early phase.