A well-differentiated endometrioid adenocarcinoma coexistent with benign and borderline-malignant endometrioid adenofibroma was found in the ovary of a 64-year-old woman. She had vaginal bleeding caused by simple hyperplasia of the endometrium due to high levels of sex steroid hormones. A FIGO stage Ia solid ovarian tumor was identified. It was composed of irregularly shaped endometriotic glands with benign and borderline malignant cytologic features embedded in abundant fibromatous stroma. Well-differentiated malignant epithelium was adjacent to these areas, but fibromatous stroma was not predominant. She was treated by surgery and three cycles of chemotherapy. This paper describes this unusual tumor and reviews the literature.