Recent studies show that colorectal tumors with high microsatellite instability (MSI-H) have increased immunogenicity and response to immunotherapy compared with microsatellite-stable (MSS) tumors. It is not yet clear whether MSI-H endometrial cancer may also benefit from these therapies. It is also unknown whether immune response is equivalent in MSI-H endometrial cancer with sporadic or inherited Lynch syndrome origins. Multiplexed fluorescent IHC was used to compare matched MSI-H ( = 60) and MSS ( = 96) endometrial cancer specimens by evaluating immune cell populations in tumor and stroma compartments. Sporadic MSI-H and Lynch syndrome-associated (LS) MSI-H endometrial cancers were also directly compared. Increased immune cells were present in stroma of MSI-H endometrial cancer compared with MSS, including granzyme B cells, activated CTLs (CD8 granzyme B ), and PD-L1 cells. Granzyme B cells and activated CTLs were also increased in the tumor compartment of MSI-H endometrial cancers. Comparing sporadic and LS MSI-H endometrial cancer showed distinct differences in immune cell populations, indicating that mechanisms underlying microsatellite instability alter immune response. Specifically, LS MSI-H endometrial cancer showed increased CD8 cells and activated CTLs in stroma, with reduced macrophages in stroma and tumor compared with sporadic MSI-H. Sporadic MSI-H had increased PD-L1 macrophages in stroma and tumor compared with LS MSI-H endometrial cancer. MSI-H endometrial cancer has increased immune cell infiltration compared with MSS endometrial cancer and the hereditary or sporadic origin of microsatellite instability impacts immune response. Clinical trials to determine the role of immunotherapy in patients with MSI-H endometrial cancer must evaluate Lynch syndrome-related and sporadic MSI-H tumors separately. .