To investigate the clinicopathological features and treatment outcomes in patients with stage I, high-risk endometrial cancer. Patients with International Federation of Gynecology and Obstetrics stage I, papillary serous, clear cell, or grade 3 endometrioid carcinoma treated between 2000 and 2012 were analyzed for the clinical and pathological factors in relation to prognosis. A total of 267 patients (stage IA; n = 175, stage IB; n = 92) were included. Among the clinicopathological features, stage and age were significant prognostic factors. The recurrence rate and overall survival for stage IB versus IA were 22.8% versus 9.1% (p = 0.003) and 149.7 months versus 201.8 months (p < 0.001), respectively. The patients >60 years of age also had a higher recurrence rate (21.7% versus 9.7%, p = 0.008) and poorer survival (102.0 months versus 196.8 months, p = 0.001) than those ≤60 years of age. Distant recurrence (64.9%) occurred more frequently than local recurrence (24.3%) and local combined with distant recurrence (10.8%) (p < 0.001). The postoperative treatment modality had no impact on tumor recurrence rate, recurrence site, or overall survival. Distant recurrence is a major cause of treatment failure in patients with stage I, high-risk endometrial cancer. However, current adjuvant treatment appeared to have little effect in preventing its occurrence.