Background: Tumors deficient in DNA mismatch repair (dMMR) exhibit a microsatellite unstable phenotype characterized by high tumor mutational burden and an immunogenic tumor microenvironment. Despite the histology-agnostic approval of pembrolizumab for advanced dMMR/MSI cancers, responsiveness of dMMR/MSI upper tract urothelial cancers (UTUC) to immune checkpoint (IC) blockade remains largely unknown. Methods: Consecutive records of patients (pts) from a single institution with locally advanced unresectable or metastatic dMMR/MSI UTUC who received IC therapy were analyzed. The primary endpoint was assessment of objective response rate (ORR) using RECIST v1.1. Secondary endpoints were progression-free survival (PFS) and overall survival (OS) using the Kaplan-Meier technique. dMMR/MSI status was evaluated by immunohistochemistry (IHC) and/or polymerase chain reaction (PCR). Results: Ten pts were identified with locally advanced unresectable (N = 3) or metastatic (N = 7) dMMR/MSI UTUC who received therapy with IC blockade (pembrolizumab = 7, nivolumab = 2, atezolizumab = 1). Median age was 65.5 (range = 46 – 90). Six pts were male. Seven pts had germline dMMR. MSI was detected by PCR in three pts and dMMR by IHC in seven pts (PMS2/MLH1 loss = 4, MSH2 loss = 1, MLH1 loss = 1, MSH6 loss = 1). Five pts received systemic chemotherapy (2 cisplatin based, 1 carboplatin based, 2 other) prior to IC therapy with two pts (40%) achieving partial response (PR). At a median follow-up of 15.5 months (range: 2 – 43 months), all pts were alive, and none experienced disease progression. PFS and OS at 15.5 months were 100%. The observed ORR was 90% (CI, 55.5%, 99.8%), including 8 pts who achieved complete remission (CR). The median time to best response was 4 months (range: 2 – 8 months). Toxicity leading to treatment discontinuation: 1 (grade 3) pancytopenia, 1 (grade 2) pneumonitis, 1 (grade 2) SICCA-like symptoms. Conclusions: Immunotherapy with IC inhibitors demonstrates excellent clinical activity in advanced dMMR/MSI UTUC. Further studies integrating these agents earlier in the disease course are warranted in this rare but important subgroup. Given the extremely high complete response rate in this population consideration of preference to IC therapy as initial therapy should be entertained if these findings are validated.