Here, we determined the frequency of microsatellite instability (MSI) and the impact of MSI-high (MSI-H) on clinical outcomes of Thai patients with endometrial cancer (EC). Tissue samples of 110 Thai patients with EC, who had undergone surgical staging, were tested for mismatch repair (MMR) gene deficiency, and the patients were grouped into MSI-H and MSI-stable (MSI-S) groups; 24.5% had MSI-H. Unlike MSI-S group patients, MSI-H group patients had synchronous and metachronous cancer. They showed better 3-year disease-free survival (DFS) than those in the MSI-S group (