We studied a case of intramural müllerian adenosarcoma of the uterus in a 51-year-old woman. The tumor measured 4 cm in diameter, was located in the lateral wall of the uterine body, and was accompanied by adenomyosis. The tumor was composed of glands with no epithelial cell atypia and a sarcomatous component evidenced by cell pleomorphism and a high mitotic count. The ratios of mitotic cells to glandular cells in the tumor, in the overlying endometrium in proliferative phase, and in the adenomyomatous tissue, were 5:10,000, 20:10,000, and less than 1:10,000, respectively. These observations seem to indicate that the glandular component of the tumor is a benign, neoplastic part, and the present tumor developed in a preexisting endometriotic focus.