A rare case of imperforate hymen associated with bicornuate uterus in an infant is presented as a cause of bilateral hydroureteronephrosis and pelvic mass in infancy. The importance of postoperative radiologic evaluation for diagnosis of accompanying uterine abnormalities is introduced. A 8-month-old girl with restlessness and intermittent fever was brought to the daily outpatient clinic by her parents. Ultrasound exam showed bilateral grade 4 hydroureteronephrosis and a large cystic pelvic mass. Magnetic resonance scan of the pelvis revealed marked hematocolpos. A cruciate incision was made over the hymen under general anesthesia. During a 6-month followup gradual resolution of bilateral hydroureteronephrosis was documented. Although the details of the uterine anomaly were obscured in preoperative imaging, postoperative US and MR demonstrated bicornuate uterus. Postoperative pelvic radiologic examination is highly recommended to verify the resolution of hematocolpos and to screen for any concomitant anomalies that can have long-term clinical significance.