Objective To investigate the presence and significance of Th9 cells and the related transcription factor ( PU.1 ) and cytokine ( IL-9 ) in peripheral blood of patents with hashimoto thyroiditis ( HT) .Methods Thirty patients with HT and thirty age/gender matched healthy subjects were recruited in this study.The peripheral blood and serum samples were collected from each subject.The percentages of Th9 cells and the transcriptional levels of PU.1 in peripheral blood mononuclear cells ( PBMCs) were meas-ured by flow cytometry analysis and real-time RT-PCR.The concentrations of IL-9, the functions of thyroid and the titers of thyroid-specific autoantibodies ( TPOAb and TgAb) in serum samples were detected by using enzyme-linked immunosorbent assay ( ELISA ) and electrochemiluminescence immunoassay analysis ( ECLIA) .Results Compared with healthy subjects, the percentages of Th9 cells and the expression of PU.1 at mRNA level in PBMCs and the concentrations of IL-9 in serum samples were all significantly in-creased in patients with HT [(1.49±0.68)%vs (0.87±0.24)%], 4.91±2.14 vs 1.66±0.52, (26.90± 7.74) pg/ml vs (16.71±5.87) pg/ml, all P<0.01).Serum concentrations of IL-9 were positively correla-ted with the percentages of Th9 cells (r=0.419, P=0.021).Moreover, the percentages of Th9 cells were positively correlated with the titers of TPOAb and TgAb in serum samples (r=0.394, P=0.032;r=0.457, P=0.011) of patients with HT.Conclusion The levels of Th9 cells and the related cytokine IL-9 were in-creased in the peripheral blood of patients with HT.A positive correlation was found between the percentage of Th9 cells and the titers of thyroid-specific autoantibodies.This study indicated that Th9 cells might be in-volved in the pathogenesis of autoimmune damage in thyroid.