BACKGROUND: The appropriate uses of lymph node dissection (LND) and adjuvant radiation therapy (RT) for Stage I endometrial cancer are controversial. We explored the impact of specific RT modalities (whole pelvic RT [WPRT], vaginal brachytherapy [VB]) and LND status on survival. MATERIALS AND METHODS: The Surveillance Epidemiology and End Results dataset was queried for all surgically treated International Federation of Gynecology and Obstetrics (FIGO) Stage I endometrial cancers; subjects were stratified into low, intermediate and high risk cohorts using modifications of Gynecologic Oncology Group (GOG) protocol 99 and PORTEC (Postoperative Radiation Therapy in Endometrial Cancer) trial criteria. Five-year overall survival was estimated, and comparisons were performed via the log-rank test. RESULTS: A total of 56,360 patients were identified: 70.4% low, 26.2% intermediate, and 3.4% high risk. A total of 41.6% underwent LND and 17.6% adjuvant RT. In low-risk disease, LND was associated with higher survival (93.7 LND vs. 92.7% no LND, p < 0.001), whereas RT was not (91.6% RT vs. 92.9% no RT, p = 0.23). In intermediate-risk disease, LND (82.1% LND vs. 76.5% no LND, p < 0.001) and RT (80.6% RT vs. 74.9% no RT, p < 0.001) were associated with higher survival without differences between RT modalities. In high-risk disease, LND (68.8% LND vs. 54.1% no LND, p < 0.001) and RT (66.9% RT vs. 57.2% no RT, p < 0.001) were associated with increased survival; if LND was not performed, VB alone was inferior to WPRT (p = 0.01). CONCLUSION: Both WPRT and VB alone are associated with increased survival in the intermediate-risk group. In the high-risk group, in the absence of LND, only WPRT is associated with increased survival. LND was also associated with increased survival.