Though malignant transformation of endometriosis has been documented, malignancy arising from extragonadal endometriosis is rare. We present the case of a 39-year-old woman with abdominal pain and fullness after menstruation. Evaluation revealed a cul-de-sac mass and CA-125 level of 1048 U/ml. A hysterectomy, bilateral salpingo-oophorectomy, and omentectomy were performed. Endometrioid adenocarcinoma with a clearly defined transition zone from endometriosis to adenocarcinoma was noted histologically. Adjuvant chemotherapy and GnRH agonist treatment was administered. Serum CA-125 level was 1.51 U/ml 19 months after completion of treatment. Patients with endometriosis and elevated CA-125 levels should be managed aggressively and CA-125 levels monitored until they have normalized.