A case of a 70-year-old woman with endometrioid adenocarcinoma of the ovary with functioning stroma is presented. The symptom was postmenopausal bleeding. The preoperative level of serum estradiol was as high as 162.4 pg/mL, and serum gonadotropin levels were suppressed. The endometrial tissue showed hyperplastic changes. The surgical specimens consisted of a multilocular cystic ovarian tumor of 95 mm in diameter and an enlarged uterus. Histologically, the tumor was composed of proliferating, atypical, columnar cancer cells resembling early secretory endometrial cells, and condensation of plumed stromal cells resembling theca lutein cells. The diagnosis of endometrial adenocarcinoma of the ovary with functioning stroma was made. After surgery, the serum levels of estradiol decreased and of follicle-stimulating hormone increased. Almost all types of ovarian tumor have been reported to be associated with endocrine abnormalities. Mucinous epithelial ovarian tumors most commonly present with estrogenic stroma, although the frequency of endometrioid adenocarcinoma with functioning stroma is very low. Here, a rare case with the patient's clinical course and histopathologic findings is reported.