The results of therapy and patterns of failure were analyzed for 60 patients with Stage I, Grade 3 endometrial cancer seen at Yale-New Haven Hospital between 1960 and 1980. Fifty-eight patients were treated with a combination of surgery and radiation; one was treated with surgery only; and one received radiation only. The overall absolute 5-year survival rate was 72.9% with poorer prognosis noted for patients greater than 65 years of age, older at time of their menopause, and with Stage IA disease. Of the 14 patients who recurred, distant sites were involved in 93% (13/14), with the lung the most common site of distant failure (5/14), followed by the upper abdomen (4/14). Pelvic sites were involved in 43% (6/14) of the treatment failures. The use of pelvic external beam radiation resulted in a reduction in pelvic recurrences, but did not improve overall survival. The predominance of distant failures despite pelvic radiation suggests the possibility of early vascular and transcoelomic spread in Stage I, Grade 3 endometrial adenocarcinomas. Thorough exploration of the upper abdomen, paraaortic nodes, and the obtaining of pelvic washings for cytology at the time of initial surgery, are recommended in addition to chest CT scans to help identify those patients with occult metastases. Prospective randomized trials in Stage I, Grade 3 patients employing adjuvant cytotoxic chemotherapy, hormonal therapy, and/or whole abdominal-pelvic radiation, should be considered in an attempt to improve survival in high-risk patients.