Objective: To analyse the effect of differentiation on disease-free survival (DFS) and overall survival (OS) in patients with stage I adenocarcinoma of the endometrium. Patients and methods: From 1979 to 1995, 350 patients with FIGO stage IA-IC with well (G1), moderately (G2) or poorly (G3) differentiated tumors were treated with surgery and high dose-rate brachytherapy with or without external radiation. Median age was 65 years (39-86 years). Results: The 5-year DFS was 8863% for the G1 tumors, 7764% for the G2 tumors, and 6767% for the G3 tumors (P50.0049). With regard to the events contributing to DFS, the 5-year cumulative percentage of local relapse was 4.6% for the G1 tumors, 9.0% for the G2 tumors, and 4.6% (P50.027) for the G3 tumors. Cumulative percentage of metastasis was 1.4, 6.3 and 7.2% (P,0.001), respectively, whereas percentages of death were 6.0, 7.9 and 20.7% (P,0.001). The 5-year OS was 9163, 8364 and 7667%, respectively (P50.0018). In terms of multivariate hazard ratios (HR), the relative differences between the three differentiation groups correspond to an increase of 77% of the risk of occurrence of either of the three events considered for the DFS (HR51.77,), (P50.078) for the G2 tumors and of 163% (HR52.63,), (P50.009) for the G3 tumors with respect to the G1 tumors. The estimated relative hazards for OS are, respectively, in line with those for DFS: HR51.51 (P50.282) for the G2 tumors; and HR53.37 (P50.003) for the G3 tumors. Conclusion: Patients with grade 1 tumors are those least exposed to either local relapse, metastasis, or death. In contrast patients with grade 2 tumors seem to be at higher risk of metastasis, whereas patients with grade 3 tumors appear at higher risk of death. Since we have looked at the first of three competing events (local relapse, metastasis and death), this suggests that patients with grade 3 tumors probably progress to death so fast that local relapse, if any, cannot be observed.