Acute urinary retention in two adolescent girls with hematometrocolpos caused by imperforate hymen are reported. The accumulation of the menstrual blood in the vagina and uterus may form a mechanical effect on the urethra and bladder and lead to the obstructive urinary symptoms. Hymenotomy created individually by CO2 laser and cruciate incision was performed. The two patients were discharged on the same day after operation and made an uneventful recovery.