A subset of endometrial adenocarcinomas (EACs) exhibit microsatellite instability and have deficient DNA mismatch repair (dMMR). The overall aim of the study was to compare the spectrum of mutations in endometrioid-type EAC with and without dMMR by using a clinically validated next-generation sequencing assay. We retrospectively identified 19 EACs with known mismatch repair status that had undergone targeted sequencing of a panel of cancer-related genes. The mismatch repair status was ascertained by immunohistochemistry against MLH1, PMS2, MSH2, and MSH6 mismatch proteins. Somatic mutations in EAC with dMMR were compared against those in cases with proficient MMR (pMMR). The dMMR EAC showed a normalized mean of 66.6 mutations/Mb per case compared with pMMR EAC with a mean of 26.2 (P<0.05). The most commonly mutated genes were PTEN (89% of dMMR, 50% of pMMR), PIK3CA (67% vs. 40%), ATM (89% vs. 40%), and FLT3 (67% vs. 50%). The transition/transversion ratio was 4.7 versus 2.8 for the dMMR and pMMR cohorts, respectively (P<0.05). Copy number variant analysis did not demonstrate significant differences between the dMMR and pMMR cohorts and was not correlated with histologic grade of EAC. In conclusion, tumorigenesis of EAC in the context of dMMR demonstrated heavier mutational burdens and higher transition/transversion ratio. The spectrum of genetic alterations can potentially help identify cases with microsatellite instability phenotype using next-generation sequencing data from a targeted cancer gene panel.