A case of carcinosarcoma of mesonephric origin in a 58 year old woman is reported. A cystic tumor with a solid area, measuring 14 cm in greatest diameter, was detected in the pelvic cavity by computerized tomography and ultrasound. Although it was diagnosed as an ovarian cancer for surgical removal, it was found to be entirely located in the myometrium of the left lateral wall of the uterine body and neither ovary was remarkable. Histologically the tumor was composed of epithelial and sarcomatous components. The former showed low papillary pattern, crowded solid nests and cords of cells, and focal tubular structures. The latter showed a solid growth pattern with differentiation to leiomyosarcoma. In the uterine cervix, a 1.2 cm mesonephric (Gartner's) cyst was found. Neither neoplastic lesions nor endometriosis were identified in the cervix, endometrium, fallopian tubes or ovaries. Based on the histologic features and the specific location of the tumor, the coexistence of Gartner's cyst, and the normal appearance of the endocervical mucosa as well as the endometrium, it was diagnosed as a mesonephric carcinosarcoma. The serum levels of carcinoembryonic antigen, CA125, CA19-9, and CA72-4 were within normal ranges in the clinical course. The patient died of disease 8 months after surgery.