Malignant transformation is an infrequent complication of endometriosis. As endometriosis is an ectopic endometrium, hyperestrogenism may cause hyperplasia or transformation into cancer. We describe a case of a 68-year-old woman who underwent total abdominal hysterectomy and bilateral salpingo-oophorectomy for endometriosis. She was subsequently placed on estrogen-only replacement therapy. She presented with left-sided pelvic mass and shortness of breath. Computed tomography of chest, pelvis, and abdomen, demonstrated right-sided pleural effusion and soft tissue mass in the pelvis. Pleural effusion was tapped and biopsy from the peritoneal mass showed metastatic adenocarcinoma; immunohistochemistry findings favored endometrioid adenocarcinoma. She was treated by 6 cycles of Carboplatin/Paclitaxel and responded well. Unopposed estrogen stimulation may lead to premalignant or malignant transformation in the residual foci of endometriosis. Therefore, the addition of progestins to estrogen replacement therapy should be considered in women who have undergone hysterectomy with oophorectomy due to endometriosis.