A 22-year-old women (gravida 0 and para 0) visited our gynecology department with the chief complaint of a foul-smelling vaginal discharge with intermenstrual bleeding. The patient denied fever, chills, nausea, vomiting, or diarrhea. Past medical and surgical history was unremarkable. Menarche had occurred at the age of 13 years. She still had some brownish discharge although her last menstrual period was 2 weeks ago. She had no complaints of dysmenorrhea, dyspareunia, or dysuria. Physical examination revealed no tenderness or palpable masses in the abdomen. Pelvic examination demonstrated a patent cervix and vagina with purulent discharge and bulging of the right lateral vaginal wall (
Fig. 1A). The discharge was cultured for
Neisseria gonorrhoeae,
Chlamydia trachomatis, and
Trichomonas vaginalis, but the results were negative. Laboratory tests revealed leukocytosis (18,220/mm
3), C-reactive protein level of 1.18 mg/dL, and normal renal function. Transvaginal ultrasound images revealed uterus didelphys with a hypoechoic heterogeneous cystic mass measuring 4.8×5 cm behind the bladder. The right kidney was not visualized on subsequent renal ultrasound. Abdominopelvic MRI was performed to better identify the pelvic anatomy and verify the absence of the right kidney. This study confirmed the ultrasound findings of uterus didelphys with a distended right hemivagina measuring 3.1×3.5×4.8 cm (
Fig. 2A), suggesting a turbid fluid collection (
Fig. 2B), and right renal agenesis (
Fig. 2C). The patient was diagnosed with HWWS complicated with pyocolpos. Resection of the vaginal septum was planned and prophylactic antibiotics were administered before surgery. Initially, a puncture was made into the right vaginal wall to identify the obstructed vaginal septum before the resection began. After confirmation of the septum, it was incised with electrocautery, and 200 mL of foul-smelling purulent material was drained (
Fig. 1B). A specimen was sent for culture and sensitivity testing. The vaginal septum was widely excised until the cervix was reached (
Fig. 1C). Laparoscopic exploration revealed uterine didelphys, powder-burn appearing endometriosis on the serosal surface of the uterus (
Fig. 1D), and pelvic adhesions (
Fig. 1E). The patient underwent laparoscopic ablation of the endometrial implants. Cultures from the specimen grew group B beta-hemolytic streptococcus, for which the patient was given a 7-day course of amoxicillin-clavulanate. She was discharged 2 days after surgery, having had an uneventful recovery.