Backgrounds: Interleukin (IL)-10-producing regulatory B cells (B10 cells) have been shown to ameliorate psoriasis in mice. Human B10 progenitor cells are characterized as CD19(+)CD24(hi)CD38(hi) B cells that exert their regulatory functions via the production of IL-10. However, the role of B10 cells in the pathogenesis of psoriasis remains unclear., Objectives: We examined B10 cells in patients with psoriasis and healthy controls., Methods: Peripheral blood mononuclear cells were isolated from psoriasis patients without a history of receiving any immunosuppressants during the 6-month period before enrollment in the study. Using flow cytometry, we determined the frequencies of blood B cell subsets, B10 progenitor cells, and B10 cells for 31 patients with psoriasis and 26 healthy controls., Results: Both psoriasis patients and healthy controls showed similar frequencies of total B cells, IgD(+)CD27(-) naïve B cells, and IgD(-)CD27(+) memory B cells. However, the frequency of CD19(+)CD24(hi)CD38(hi) B10 progenitor cells was significantly higher in patients with psoriasis than in the healthy controls. In contrast, the frequency of B10 cells in patients with psoriasis was significantly lower than that in healthy controls. Furthermore, treatment with immunosuppressants resulted in a decrease in B10 progenitor cells and an increase in B10 cells., Conclusion: B10 progenitor cells were increased, while IL-10-producing regulatory B10 cells were decreased in patients with psoriasis, suggesting that B10 cells may be functionally impaired in patients with psoriasis., (Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.)