Abstract Three cases of tumor of the endometrium are reported which fulfill the criteria of malignancy. Two of them had massive recurrence after operation and one metastasized. They deserve the term “sarcoma” rather than endometriosis. They were peculiarly sensitive to radiation. One case of sarcoma of the endometrium is reported which had a histologic picture unlike that of the two previous cases and contained cartilage. This should probably be called a teratoma, or mixed-cell type of tumor. One case of so-called carcinosarcoma of the endometrium is reported. This tumor metastasized to the vagina as a true adenocarcinoma suggesting that the sarcoma-like tissue of the stroma was not malignant.