This report describes a case of high-grade stromal sarcoma occurring in a 53-year-old woman. Preoperative and pathologic findings showed up a large tumoral nodule confined to the uterus. The patient underwent a total abdominal hysterectomy, bilateral salpingo-oophorectomy, omentectomy and dissection of pelvic and para-aortic lymph-nodes; successively she underwent pelvic and abdominal radiotherapy and chemotherapy, which was discontinued because of leukopenia. Histologically the tumor was made up of cells resembling endometrial stromal cells with only mild cytologic atypia and had up to 24 mitoses x 10 HPF in the most cellular areas. Immunohistochemically it showed a diffuse positivity for vimentin and a focal positivity for muscular actin and desmin. The patient died of this disease nine months postoperatively. These features underline that a uterine sarcoma with histological features similar to a low-grade endometrial stromal sarcoma but with high mitotic rate may behave as a very aggressive tumor even when diagnosed at the initial stage; the immunohistochemical data indicate that these tumors may show smooth-muscle differentiation.