Noncommunicating uterine horns are rare, occasionally presenting with functional endometrial cavities. Surgical removal of the noncommunicating horn is commonly performed to prevent endometriosis in these patients. A 41-year-old woman with a unicornuate uterus and noncommunicating uterine horn presented with a three-month history of right-sided pelvic pain. She had previously undergone multiple assisted reproductive technique attempts with superovulation and supraphysiologic serum estradiol levels and no apparent symptomatology or evidence of hematosalpinx during laparoscopy. Shortly after completing a donor oocyte recipient cycle, she developed acute right-sided pelvic pain. Diagnostic laparoscopy and subsequent laparotomy confirmed a right hematosalpinx and hematometra of the noncommunicating horn, with stage III endometriosis. Consideration of prophylactic resection of a noncommunicating uterine horn with a cavity should be considered in an asymptomatic, reproductive-age patient with this rare müllerian anomaly.