Objective — to investigate the MSI phenotype of urothelial bladder cancer (BC) by immunohistochemistry (IHC) and to assess its relationship to the prognostic factors of the disease and PD-L1 status of BC. Material and methods. Using the surgical and biopsy materials obtained from 50 patients with BC during examination and treatment, the investigators studied the MSI phenotype by IHC. A system was proposed to assess the IHC expression of MSI proteins, by taking into account the intensity of nuclear staining and the area occupied by tumor cells with stained nuclei. Results. An analysis of the results of an IHG study of MSI protein expression revealed a high direct correlation between the nuclear staining intensity of tumor cells and the percentage of tumor area occupied by the latter. The lack/decrease of the expression of the studied proteins was associated with the stage (T) and the presence of a high-grade tumor. The heterogeneous expression of the studied proteins (PMS2, MLH1, and MSH6) was noted to be 10, 30, and 40%, respectively. Conclusion. A high direct correlation was observed between the nuclear staining intensity of tumor cells and the percentage of the area occupied by the latter. There was a relationship between the lack and/or decrease of the expression of MSI proteins (mainly PMS2 and MLH1 and to a lesser extent MSH6) and the grade of BC, as well as stage T; there was a tendency to decrease the expression of the studied proteins in the area of invasive tumor growth, which confirms the prognostic role of MSI proteins. The most pronounced heterogeneity of IHC expression was noted with MSH6; the least one was seen with PMS2. There was a predominance of high-grade surface carcinomas (T1) among the heterogeneously stained tumors. In the case of positive PD-L1 status, there was decreased PMS2 and MLH1 expression and pronounced MSH6 expression; no significant relationship was found due to the small number of these cases. The authors consider it necessary to conduct further studies of the relationship between the MSI phenotype and the PD-L1 status of BC.