A 16-year-old girl presented with primary amenorrhea and had had cyclical abdominal pain for almost a year. At examination we observed a painful mass in the lower abdomen and normal secondary sex characteristics. Perineal examination showed a bluish bulging hymen. Transabdominal ultrasonography revealed a dense mass in the pelvis measuring about 12×11 cm. We diagnosed an imperforate hymen with haematocolpos and haematometra. The hymen was opened surgically and a large quantity of menstrual blood was drained from the vagina and uterus. Postoperative recovery was normal without any pain. The patient now menstruates regularly. An imperforate hymen occurs in 0.05% of women. It is important to be aware of this while examining a female adolescent presenting with cyclical abdominal pain and primary amenorrhea. Late discovery of an imperforate hymen may lead to pain, infections, hydronephrosis and endometriosis with subfertility as a possible consequence.