The development of stromal sarcomas on the foci of endometriosis is extremely rare and the differential diagnosis from other tumors of myogenic, vascular, hemopoietic or epithelial origin may present great diagnostic difficulties. We investigated the clinicopathological and immunohistochemical characteristics of 4 cases of endometrial stromal sarcoma that developed on endometriotic foci of the uterus, vagina and omentum. Thye were classed as high grade (1/4) or low grade (3/4) malignant potential tumors, according to their mitotic activity. Immunohistochemically these tumors gave a positive reaction to vimentin, but were negative to desmin, smooth muscle actin, factor VIII, EMA and LCA. These characteristics permit their identification and a proper therapeutic approach.