Background: MSI-H cancers are responsive to immune checkpoint blockade (ICB), but nearly half of all patients experience primary or early treatment resistance. Activation of the WNT/B-Catenin pathway can lead to immune exclusion and may drive resistance to ICB. Methods: 12 patients had stage III (N = 1) or IV (N = 11) MSI-H GI tract (small bowl, colon, or rectal) cancers. Blood samples were obtained after (N = 5) or during (N = 5) ICB. 2 patients did not receive ICB. Blood samples from 8 patients with microsatellite stable (MSS) metastatic colorectal cancer were included as controls. The Guardant Health (Redwood City, CA) Omni 2.0 mb panel was used to analyze cfDNA. We analyzed MSI-H status, TMB, and mutations within the WNT/B-Catenin pathway, including APC, RNF43 and CTNNB1. Results: Of 12 patients with MSI-H GI cancers, 1 sample failed enrichment due to hemolysis. MSI-H was not detected in 2 patients with a history of MSI-H in tissue; however these patients had a complete response to ICB at the time of blood collection. The Omni panel identified MSI-H in the remaining 9 patients with MSI-H disease in tissue. Among 8 control patients with MSS disease in tissue, MSI-H was not detected. Median TMB (mutations/Mb) was greater for MSI-H specimens (109; range 30-807) than for MSS specimens (13; range 6-24). All 8 patients with MSS GI cancers were identified to have APC mutations, and none were found to have CTNNB1 or RNF43 mutations. Of 9 evaluable MSI-H GI cancers, 2 had APC mutations alone. The remaining 7 carried RNF43 mutations (G659fs). All patients with RNF43 mutations were found to have disease progression while on ICB. Among these 7 patients with RNF43 mutations, 6 had additional mutations in APC or CTNNB1. Conclusions: Blood based genomic profiling can identify MSI-H cancers. Patients with MSI-H cancers resistant to ICB in this cohort have mutations in RNF43 as well as additional mutations in APC or CTNNB1, suggesting that co-activation of the WNT/B-Catenin pathway may be biologically important. Further study of the role of WNT/B-Catenin pathway activation in ICB resistance will be pursued using tumor tissue from this cohort.