Undifferentiated carcinoma of the endometrium is a high grade carcinoma and under-recognised, frequently diagnosed as grade 3 endometrioid carcinoma. We have found that undifferentiated carcinoma represents 9% of all endometrial carcinomas; it is composed of solid sheets of epithelial cells and in most cases only 5-10% of the cells are positive for keratin. The recognition of undifferentiated carcinoma is extremely important when this neoplasm is associated with a grade 1 or grade 2 endometrioid adenocarcinoma. In this situation it is important to diagnose the solid areas of carcinoma as undifferentiated and avoid evaluating them as the solid component of endometrioid carcinoma because there is significant difference between the excellent prognosis of grade 2 endometrioid carcinoma, the intermediate prognosis of grade 3 carcinoma, and the poor prognosis of undifferentiated carcinoma, which in cases of association with differentiated areas should be diagnosed as dedifferentiated endometrioid carcinoma.