Background: Up to now, endometrial cancer (EC) treatments are mainly represented by surgery followed by adjuvant chemotherapy or radiotherapy. The updated guidelines give a 2A recommendation for the use of hormone therapy only in advanced low-grade ECs, underlying the need for more data on the role of hormone therapy in the adjuvant setting. Methods: The clinicopathological data of 158 early-stage EC patients was retrospectively collected. A Ki-67 cut-off value of 40% was established based on literature data. Disease-free survival (DFS) and Overall survival (OS) were evaluated. Results: Results: Multivariate analysis of DFS and OS showed a significantly increased risk of progression in patients with >40% Ki-67 [HR = 3.13 (95% CI; 1.35–7.14); p = 0.007] and a significantly higher relative risk of death [HR = 3.70 (95% CI; 1.69–8.33); p = 0.001]. The predictive role of the Ki-67 index was highlighted by the clinical benefit of adjuvant hormone in patients with high Ki-67. Conclusions: Our results suggest a positive role of the Ki-67 index as a prognostic and potentially predictive marker in EC, although further studies are warranted to reach a definitive conclusion.