BACKGROUND: CD4+CD25+ regulatory T cells (Tregs) are important for the maintenance of immune homeostasis by virtue of their ability to control T-cell proliferation in the peripheral blood (PB). We recently demonstrated that the prevalence of Tregs is decreased, whereas that of Th17 cells is increased, in the PB and decidua samples of patients with unexplained recurrent miscarriage (RM). In this study, we investigated whether the cytokine production of Th17 cells can be suppressed by the Tregs and elucidated the mechanism by which Tregs exert this suppressive effect. METHODS: Flow cytometry was used to analyze the surface phenotype and cytokine production of Th17 cells in the PB of women with unexplained RM (n = 17) and healthy women in early stages of pregnancy who underwent elective abortion (n = 20). The suppressive ability of Tregs on Th17 cells was assessed in in vitro co-cultures and transwell experiments. The amount of secreted interleukin-17 (IL-17) in the supernatants was measured by enzyme-linked immunosorbent assay (ELISA). The inhibitory activity of transforming growth factor- (TGF-) and IL-10 on IL-17 expression in CD4+ T cells was assessed using ELISA. RESULTS: The proportions of IL-17-positive CD4+ T cells, CC chemokine receptor type 6 (CCR6)-positive CD4+ T cells and CCR6 expression of IL-17-positive CD4+ T cells were higher in the PB samples of patients with unexplained RM than in PB of healthy control subjects. In vitro, Tregs could inhibit the expression of IL-17; more Th17 cells were inhibited in the control group than in the unexplained RM group. High-dose TGF- inhibited the expression of IL-17, whereas IL-10 inhibited IL-17 expression in a dose-dependent manner. CONCLUSIONS: IL-17 expression can be inhibited by Tregs. The suppressive activity of Tregs on Th17 cells was decreased in patients with unexplained RM. The ability of Tregs to suppress cytokine secretion might be effected by a cell-cell contact. TGF- and IL-10 could inhibit the expression of IL-17. © 2010 The Author. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved.