Ten percent of all patients with endometrial carcinoma have Stage III disease at the time of presentation. The management, the features of their disease, and their prognosis are quite different than those of patients with Stage I disease. This report is based on 37 patients with Stage III carcinomas. For their treatment, a program of definitive radiation therapy was applied. Eleven patients had a prior total abdominal hysterectomy (TAH) and bilateral salpingo-oophorectomy (BSO). On the basis of the tumor extension, three main patterns were identified: 1) downward into the vagina or the vagina and the cervix; 2) lateral into the parametrium and the pelvic wall; and 3) to the ovaries. This classification carries therapeutic and prognostic significance. Ovarian extension has the best prognosis when treated by TAH and BSO followed by postoperative radiotherapy. Extension to the vagina or to the vagina and the cervix can be treated successfully by a combination of external beam and local radium placements. Patients with pelvic wall extension have the poorest prognosis. They comprise maore than 50% of all cases with Stage III tumors and have exhibited persistent or recurrent disease even when treated at high dose levels. The cumulative survival rates for the entire stage were 50% at the end of the first year, 32% at the end of the second year, and 25% at the end of the fifth year.