Objective To investigate the influences of IL-17 on regulatory T ( Treg) cells during Chlamydia trachomatis infection. Methods Wild-type ( WT) C57BL/6 mice and IL-17-/- mice were in-tranasally injected with 1×103inclusion forming units (IFU) of Chlamydia muridarum (Cm) to establish the mouse model of Chlamydia trachomatis respiratory tract infection. Mouse spleen and lung single cells were prepared. The percentages of CD4+CD25+T and CD4+CD25+Foxp3+T cells were detected by flow cytome-try. Expression of Foxp3 and TGF-β at mRNA level in lung was detected by RT-PCR. The levels of IL-10 in bronchoalveolar lavage fluid samples were detected by ELISA. Results Compared with the WT mice, the IL-17-/- mice had higher percentages of CD4+CD25+T and CD4+CD25+Foxp3+T cells in spleen and lung on the third day of Cm infection. Both of the expression of Foxp3 at mRNA level in lung and the secretion of IL-10 in bronchoalveolar lavage fluid were increased in IL-17-/- mice as compared with those in WT mice. No significant difference in the expression of TGF-β at mRNA level in lung tissues was found between the two groups. Conclusion IL-17 might inhibit the proliferation of Treg cells and the secretion of IL-10 in the very early stage of Cm respiratory tract infection.