We report on the pathologic findings in primary tumors and metastases in 203 cases of stage I and II endometrial carcinosarcoma (malignant mixed mesodermal tumor) subjected to hysterectomy and staging laparotomy. Metastases were studied in 40 of these cases, including 34 with positive findings in the pelvic and/or para-aortic lymph nodes. Features of the stromal component of the primary tumors, including grade, mitotic index, and the presence and types of heterologous elements, showed no relation to the presence of metastases at operation. High-grade, serous, and clear cell carcinomatous components, on the other hand, were associated with a higher frequency of metastases, as were deep myometrial invasion, lymphatic or vascular space invasion, and involvement of the isthmus or cervix. The current concepts of histogenesis and differentiation of these tumors are discussed, and the suggestion is made that they might represent metaplastic carcinomas.