Background: Prospective comparison of different methods for recognition of LS associated EC is lacking. Microsatellite instability (MSI) has been observed to be inconsistent as a prognostic factor in EC. The aim of this study was to analyze the impact of MSI and Mismatch Repair (MMR)-deficiency on LS identification and prognosis in a prospective series of EC treated in one single institution. Methods: Consecutive and prospective EC patients were included. Clinical data, family history of cancer and criteria for LS were evaluated. MSI and MMR protein expression were performed. When loss of MMR proteins and/or high MSI (MSI-H) was observed, complete sequencing and large genomic rearrangement analysis of germline MLH1, MSH2, MSH6 or PMS2 was done once MLH1 methylation was excluded. Results: Seventy-seven women diagnosed with EC were enrolled from 2009 to 2012 with a median age of 62 (31-87), 84% histological type I, 16% type II and FIGO stages I 83%, II 2.8%, III-IVA 7% and IVB 7%. Among which, 6.6% fulfilled revised Bethesda criteria (rBC), 5.3% criteria defined by the Society Gynecologic Oncology (>20%-to-25% risk of LS) (SGO) and 5.3% Amsterdam II criteria (AII). Seventy-four underwent surgery with curative-intent. Tumor tissue was unavailable in 5 cases for molecular analysis. MSI-H was observed in16 cases (22%) and 18 (24%) had loss of expression of MLH1 (14), MSH2 (2), MSH6 (1) or PMS2 (1). Nine MLH1 hypermethylation, 10 K-RAS and no BRAF mutations were identified. Four (5.5%) pathogenic MMR germline mutations were recognized: 2 MSH2, one MSH6 and one PMS2. One patient (25%) with MSH6 mutation did not fulfil rBC, SGO nor AII criteria and was identified both by MSI-H and loss of MSH6 expression. Median follow-up was 23.4 (0.2-76.3) months. No correlation between MSI-H, MMR gene mutation and relapse was observed in stage I-IVA EC. Conclusions: Based on these results, rBC, AII and SGO criteria for identification of LS in EC do not identify 25% of germline MMR gene mutations. No correlation with relapse and MSI-H could be identified.