Endometriosis is a common clinical disorder that shares certain characteristics, metastasis and recurrence, with malignant neoplasms. Most malignant ovarian tumors arising from endometriosis are clear cell carcinoma or endometrioid adenocarcinoma. Few reports exist of sarcoma associated with endometriosis, and even fewer exist of multiple types of malignancies occurring simultaneously. Here, we report the case of a 32-year-old woman who presented with infertility and a pelvic mass. She underwent exploratory laparotomy and bilateral salpingo-oophorectomy. She was then referred to our institution for treatment recommendation. The pathologic findings revealed bilateral endometrioid adenofibroma of low malignant potential, which was associated with endometrioid intraepithelial carcinoma in the left ovary and high-grade sarcoma in the right ovary. Both tumors seemed to have arisen from endometriosis. She was treated with 75 mg/m2 of doxorubicin and 10 g/m2 of ifosfamide every three weeks for eight courses. She was later found to have bilateral brain metastases, which were resected and treated by whole-brain irradiation. She was again treated with doxorubicin and ifosfamide. The optimal treatment for endometriosis-associated ovarian cancer depends on the type of malignancy; simultaneously occurring multiple tumor types should be treated individually.