Tumor size was determined in 142 patients with clinical Stage I endometrial cancer treated primarily by total abdominal hysterectomy, bilateral salpingo-oophorectomy, and lymph node biopsies between July 1979 and August 1988. Only 4% of patients with tumor size less than or equal to 2 cm had lymph node metastasis; this increased to 15% for tumors more than 2 cm and increased further to 35% when the entire uterine cavity was involved (multivariate P = 0.01). Five-year survival was 98% for patients with tumors less than or equal to 2 cm, 84% with tumors more than 2 cm, and 64% with tumors involving the whole uterine cavity (Mantel-Cox P = 0.005). For endometrial cancer patients with Grade 2 tumors and less than one-half myometrial invasion, the risk of lymph node metastasis is often considered too low to justify adjuvant pelvic radiation therapy. This intermediate-risk group is better defined by including tumor size as a prognostic factor. For this subgroup (Grade 2, less than one-half endometrial invasion) there were no lymph node metastasis associated with tumors less than 2 cm, but 18% had nodal disease when tumors were larger than 2 cm. Tumor size is an important prognostic factor that is particularly helpful in directing adjuvant radiation therapy in patients without staging lymph node biopsies.