BACKGROUND This retrospective clinical study aimed to investigate the effect of prognostic factors and adjuvant radiotherapy in patients with high-grade early-stage endometrial cancer on overall survival (OS) and disease-free survival (DFS). MATERIAL AND METHODS The medical records of patients diagnosed with high-grade, early stage (I or II) endometrial adenocarcinoma who had received adjuvant radiotherapy after surgery were reviewed. RESULTS Seventy-nine patients included 39 patients (49.4%) with stage II endometrial cancer, 25 patients (31.6%) with histologic grade 3 tumors, and 47 patients (59.5%) with endometrial cancer showing lymphovascular space invasion (LVSI). There were 45 patients (57.0%) who received external pelvic radiotherapy with an average dose of 46.0 Gy (range, 11.2-50.4 Gy), and 34 patients (43.0%) received vaginal brachytherapy (VBT) with an average dose of 21.5 Gy (range, 10-36 Gy). Multivariate analysis showed that tumor stage (HR, 4.066; 95% CI, 1.227-13.467; p=0.022) and histologic grade (HR, 16.652; 95% CI, 4.430-62.589; p<0.001) were independent predictors for OS. Increased serum CA-125 levels (HR, 1.136; 95% CI, 0.995-1.653; p=0.047) and histologic grade (HR, 3.236; 95% CI, 1.107-15.156; p=0.015) were independent predictors for DFS. Adjuvant radiotherapy was not found to be significantly associated with improved OS (HR, 1.259; 95% CI, 0.518-3.058; p=0.612) or DFS (HR, 1.056; 95% CI, 0.994-1.123; p=0.078). CONCLUSIONS This retrospective study showed that in high-grade early-stage endometrial cancer treated with postoperative adjuvant radiotherapy, independent predictors for OS were tumor stage and grade. Adjuvant radiotherapy was not associated with improved OS or DFS.