Extrauterine müllerian adenosarcoma is rare and is thought to arise from endometriotic deposits or pluripotent mesothelial and mesenchymal cells of the pelvic cavity. We presented a 23 year -old female with a large pelvic mass. Laparotomy and intraoperative consultation were performed. Histologically, mass revealed endometriosis foci and diagnostic appearance of Müllerian adenosarcoma. The possibility of adenosarcoma should be considered if an enlarging mass occurs at the side of extraovarian endometriosis. [Turk J Cancer 2009;39(4):155-157]