From 1968 to 1974, 540 patients with stage I adenocarcinoma of the corpus uteri entered a prospective clinical trial to evaluate the effect of postoperative external pelvic irradiation. After primary surgery all patients received intravaginal radium irradiation; 6000 rads was delivered to the surface of the vaginal mucosa. At the time vaginal radium was given, randomization was performed: Group A received no further treatment (controls); group B received additional high-voltage irradiation to the pelvic field with a dose of 4000 rads to the pelvic lymph nodes. During the follow-up period of 3 to 10 years a significant reduction in vaginal and pelvic recurrences was found in group B as compared with group A (1.9 versus 6.9%, P < .01). On the other hand, more patients in group B developed distant metastases than those in group A (9.9 versus 5.4%). Thus, the 5-year survival rate was not improved by external irradiation. A more detailed analysis of the series led to the conclusion that only patients with poorly differentiated tumors (grade 3), which infiltrate more than half the myometrial thickness, might benefit from additional external radiotherapy. In almost 20% of 151 consecutive patients, tumor cells were found in endothelial lined spaces. Significantly more deaths and recurrences were found among these patients compared to those without vessel invasion (26.7 versus 9.1%, P < .01).