We report the case of a 79-year-old woman with a large pelvic mass and postmenopausal bleeding, associated with hyperestrogenism. A pelvic MRI shows the presence of a large mass of 12.6 cm originating from the right ovary without signs of metastasis. A total abdominal hysterectomy with unilateral salpingooophorectomy was performed, knowing the patient underwent a left salpingooophorectomy decades ago. The pathological findings showed an ovarian clear cell carcinoma (pT1A) with associated endometrial intraepithelial neoplasia. There is convincing evidence that the production of estrogen is located in the activated ovarian stroma. This supports the view that functioning stroma of ovarian cancer can lead to hyperestrogenism and eventually endometrial cancer.