BACKGROUND Microsatellite instability (MSI) is a hallmark of specific cancers and can be diagnosed using both tissue- and liquid-based approaches. When these tissue- and liquid-based approaches give differing results, they are known as discordant or being at variance. MSI-H tumors are well-researched candidates for treatment with programmed cell death protein 1 (PD-1) inhibitor-based immunotherapy, but the efficacy of immunotherapy in MSI-H discordant endometrial cancer, especially as first-line therapy, is not yet well documented in the literature. CASE REPORT A 67-year-old woman presented with a retroperitoneal mass positive for recurrent adenocarcinoma of endometrial origin. Her stage I endometrial adenocarcinoma 7 years ago demonstrated microsatellite stable (MSS) by immunohistochemical (IHC) stain and indeterminant due to insufficient tissue by Caris Next-Generation Sequencing (NGS). She then presented with a retroperitoneal mass that was MSI-H on IHC stain and Caris NGS, as well as MSI high on liquid biopsy @Guardant360 (@G360). The patient proceeded with pembrolizumab treatment 1 year ago and has sustained a complete clinical response at the time of writing. CONCLUSIONS Our case provides further evidence for the need to retest the microsatellite stability of metastatic sites, especially after a long disease-free survival. Here, we providing a literature review of case reports and a review of studies outlining discordance of testing modalities. Our case also highlights the importance of considering the use of immunotherapy as a first-line agent in patients who may have a poor ECOG performance status, as it can significantly improve their quality of life and reduce the number of adverse effects compared to chemotherapy.