Acute urine retention can be the first clinical sign of a rabdomyosarcoma (RMS) of bladder and prostate. This is particularly evident in the age group comprised between 2 and 4 years. The Authors report their experience on three cases of 2 years old males admitted for acute urine retention. One case was diagnosed to have an prostate RMS, the other two a RMS of the bladder. The prostate RMS refused the operation dying 2 years after operation. Of the two cases of RMS of the bladder, one was treated successfully by polychemotherapy treatment, the second, after polychemotherapy underwent a partial cystectomy. The Authors conclude affirming the importance of a ultrasonography of the lower urinary tract in all patients with acute retention of urine, since this could be the first sign of a RMS of prostate or bladder.