Extrauterine mullerian adenosarcoma is rare and is thought to arise from endometriotic deposits or pluripotent mesothelial and mesenchymal cells of the pelvic cavity. Concomitant colon-rectal neoplasm was reported but the relationship between extrauterine adenosarcoma and colon tumor was not studied. We describe an extrauterine adenosarcoma with a concomitant rectal tubulovillous adenoma. The patient had a long-term history of endometriosis and unopposed estrogen therapy. Immunohistochemical study was performed to investigate the origin and nature of the adenosarcoma. Immunostaining provided the evidence to distinguish between rectal tumor and adenosarcoma. Extrauterine adenosarcoma may arise from endometriosis, and hormone replacement therapy may have some role in the malignant transformation process. Adenosarcoma should be considered in the differential diagnosis of a new pelvic tumor in a patient with a history of endometriosis.