Introduction: Myeloid derived suppressor cells (MDSCs) are one of the major components of the tumor microenvironment. The accumulation of MDSCs has been demonstrated in many types of human solid tumors. However, the relevance of this heterogeneous population in hematopoietic malignancies has only recently gained stronger attention. MDSCs are a phenotypically and functionally heterogeneous group of cells. The results of recent studies indicate that the immune dysregulation in chronic lymphocytic leukemia (CLL) affects a monocytic MDSC (M-MDSC) subpopulation. This study aimed to analyze the frequency of M-MDSCs with intracellular IL-10 and TGF-b1 expression in newly diagnosed CLL patients. We investigated the potential role of M-MDSCs in CLL by analyzing the level of IL-10 and TGF-β1 expression in circulating M-MDSCs in correlation with clinical and laboratory parameters characterizing disease activity and patients' immune status. Material and methods: Seventy CLL patients and 17 age-matched healthy volunteers were included in this study. Flow cytometric detection of Mo-MDSCs (CD14+CD11b+CD15-HLA-DR-/low) with intracellular IL-10 and TGF-c1 expression was done. Results: We found a significantly higher median percentage of M-MDSC with IL-10 or TGF-β1 expression in CLL patients than in healthy volunteers. The percentage of M-MDSC with intracellular IL-10 or TGF-β1 expression was significantly lower in CLL patients at stage 0 as compared to the stages I/II and III/IV according to Rai stages. The percentage of M-MDSC with intracellular TGF-β1 expression was significantly higher in ZAP-70-positive and CD38-positive patients compared with ZAP-70-negative and group of CD38-negative ones. There was also a significantly higher percentage of M-MDSC positive for intracellular TGF-β expression in patients carrying the 11q22.3 and/or the 17p13.1 deletion than in patients without these genetic aberrations. The percentage of M-MDSC IL-10-positive and M-MDSC TGF-β1-positive measured at the time of diagnosis was higher in patients requiring therapy as compared to patients without treatment during the observation period. Conclusion: In conclusion, we have shown that an increased percentage of M-MDSC cells producing IL-10 and TGF-β1 in CLL patients may be associated with the suppression of the immune response against CLL. It can be assumed that the increased percentage of M-MDSC with an intracellular expression of IL-10 and TGF-β1 may be used in the future as the factor defining the group of patients with shorter time to onset of treatment.Keywords: IL-10; M-MDSC; TGF-b; chronic lymphocytic leukemia; flow cytometry; genetic analysis.