Objective: Chronic Lymphocytic Leukemia (CLL) is characterized by the accumulation of CD5+CD19+ B cells in the bone marrow and peripheral blood. Recent studies indicated that expression of IL-10, AID and mir-155 which are regulated by STAT3 are increased in CLL patients. CD5+CD19+ regulator B (Breg) cells secrete IL-10 and suppress the immune system. While the CLL cells show similar immunophenotypic properties to Breg cells, they are also thought to be functionally similar. In this study, STAT3 and IL-10 levels of CLL patients were investigated. Methods: Peripheral blood samples obtained from patients (n:24) and healthy controls (n:14). Peripheral blood mononuclear cells were cultured for 48 hours in the presence and absence of CpG for IL-10 expression and cultured with and without PMA for STAT3 expression. IL-10 and STAT3 expression were analyzed with anti-CD5, anti-CD19, anti-CD38, anti-STAT3 and anti-IL-10 monoclonal antibodies by using flow cytometry. Results: Compared to healthy subjects, increased IL-10+, IL-10+CD19+, STAT3+CD19+ were obtained in lymphocyte population of patients. Increased IL-10 was showed CD19+ B cells of CLL patients. Our results showed that IL-10 levels had no significant difference between CD5+CD19+ cells, whereas STAT3 levels were found lower in patient compared to healthy controls. Conclusion: These results made us thought that the levels of IL-10 and STAT3 expression in CLL B cells is clearly different from normal B lymphocytes might have a role in the biology of CLL. It is believed that the presented data will contribute to the studies that scrutinize the similarity of CLL cells to Breg cells.