Background: An accessory uterine cavity is an unclassified anomaly of the paramesonephric ducts. It is a cause of severe cyclic pelvic pain. Pregnancy may occur in this cavity. Case: A 32-year-old female presented with recurrent cyclic pelvic pain. Previously, she had several pregnancies: one was full term and the others three ended in miscarriages. She was diagnosed with an accessory uterine cavity, based on hysterosalpingogram and laparoscopy findings. She became pregnant in the accessory cavity, required methotrexate treatment, and finally surgical excision. Pathology was confirmatory for the presence of an acessory uterine cavity. Results: Postoperatively, this patient's course was uncomplicated, and she was discharged to go home on postoperative day 1. Conclusions: Diagnosis of an accessory uterine cavity is based on normal hysterosalpingogram results and the presence of a mass lesion in one side or the other of the uterine fundus. Surgical excision relieves patients' symptoms and prevents ectopic pregnancy in that accessory uterine cavity. ( J GYNECOL SURG 31:347)