Background <p>Lymphovascular space invasion (<i>LVSI</i>) is an established prognostic factor in endometrial cancer; however, its biological and clinical impact in surgically staged early-stage cancer is unclear. This study aimed to evaluate the prognostic significance of substantial LVSI, defined per the WHO 2020 criteria as ≥5 vessel involvement, in stage I-II endometrioid endometrial cancer.</p> Methods <p>We retrospectively analyzed 215 patients with surgically staged, lymph-node negative, stage I-II endometrioid endometrial cancer. Substantial LVSI was defined as ≥5 vessel involvement; focal LVSI was classified as negative. Associations between LVSI and clinicopathological features, adjuvant treatment, and survival outcomes were evaluated. Survival analyses were performed using the Kaplan-Meier method and Cox proportional hazards models for overall survival (<i>OS</i>), disease-free survival (<i>DFS</i>), distant metastasis-free survival (<i>DMFS</i>), and locoregional recurrence-free survival (<i>LRFS</i>).</p> Results <p>Substantial LVSI was identified in 6.5% of patients. Substantial LVSI-positive patients received adjuvant radiotherapy more frequently yet demonstrated significantly higher locoregional recurrence. In multivariate analysis, age and stage were independent predictors of OS and DFS; age alone was an independent predictor of DMFS. Substantial LVSI was the only independent prognostic factor for locoregional recurrence (HR: 4.531, 95%Cl 1.453–14.132, <i>p</i> = 0.009).</p> Conclusion <p>Substantial LVSI is a strong predictor of locoregional recurrences in endometrial cancer, even in patients receiving adjuvant radiotherapy. These findings suggest that LVSI reflects an intrinsically aggressive tumor biology that current adjuvant treatment strategies may not fully control.</p>