Background: Congenital anomaly of the Mullerian duct system can result in various urogenital anomalies including uterus didelphys with blind hemivagina and ipsilateral renal agenesis. The diagnosis of this condition is usually made after menarche, but its rarity and variable clinical features may contribute to a diagnostic delay for years after menarche. Case: A 24-year-old woman presented to the emergency room of the Department of Obstetrics and Gynecology complaining of severe worsening lower abdominal pain, vomiting and pus-like vaginal discharge. Physical examination revealed acute abdomen with diffuse lower abdominal tenderness, rebound and muscular resistance. Cervical and vaginal observation was impossible because of the patient's discomfort. Bimanual gynecological examination showed high tenderness cervical motion. Transabdominal ultrasound scan was performed and the radiologist interpreted the ultrasonographic findings as existence of a pelvic mass with mixed echogenicity. The preoperative diagnosis was ruptured tubo-ovarian abscess and emergency laparotomy was performed. Free pus in the pelvis was found. Also, a double uterus with an elongated and inflammatory right fallopian tube with pus passing out of its fimbrial end was observed. Vaginal examination under general anesthesia revealed an obstructed right hemivagina with vaginal pus-like discharge from a small fistula hole on the septate vagina. The final diagnosis was uterus didelphys with unilateral imperforate right hemivagina and pyocolpos. Transvaginal resection of the vaginal septum was performed and a large amount of pus and blood was spilled out. Postoperatively, intravenous pyelography demonstrated agenesis of the right kidney. Conclusion:We demonstrated the difficulty in making a correct diagnosis of this rare congenital anomaly of the female genital tract, especially after many years since menarche. This condition should be considered to prevent misdiagnosis or suboptimal treatment and decrease morbidity and unnecessary surgical procedures.