Objective To observe Th9 cells and IL-9 level in adult patients with primary immune thrombocytopenia(ITP), and to discuss their potential roles in the pathogenesis of ITP. Methods A total of 25 newly diagnosed ITP patients and 25 sex- and age-matched healthy controls were enrolled in the present study. The percentage of Th9 cells in the peripheral blood samples of the two groups were detected by flow cytometry, expressions of IL-9, TGF-β, PU.1 and IRF4 mRNA were analyzed by real time-RCR, and IL-9 protein level was examined by ELISA. The platelet count was recorded by sysmex XE-2100. Results Compared with the healthy controls, the ratio of Th9 cells was significantly increased in ITP patients(P<0.05); and the ratio of Th9 cells in patients with PLT≤30×10⁹/L was significantly higher than that in patients with PLT>30×10⁹/L(P<0.05). IL-9 mRNA and protein expressions in ITP patients were significantly higher than those in the healthy controls, and those in patients with PLT≤30×10⁹/L were also significantly higher than those with PLT>30×10⁹/L(P<0.05). Compared with healthy controls, the mRNA expressions of IL-9, TGF-β, PU.1 and IRF4 were raised significantly in ITP patients(P<0.05). The ratio of Th9 cells and IL-9 protein level were negatively correlated with PLT in ITP patients(r=-0.428 1, P=0.032 8; r=-0.537 5, P=0.005 6, respectively). Furthermore, follow-up study of 11 ITP patients found that both Th9 cells and IL-9 protein levels took a declining tendency after effective treatment(P<0.05). Conclusion Abnormal activation of Th9/IL-9 may participate in the occurrence and development of ITP disease, which provides new clues for further understanding of ITP pathogenesis and selecting potential therapeutic targets in immune therapy of ITP.