Objective To investigate the relationship of interleukin-9 (IL-9) and the transcription factor PU.1 with chronic urticaria.Methods Thirty patients with chronic urticaria (patient group) and 30 healthy volunteers (control group) were included in this study.Venous blood samples were collected from these subjects.Enzyme-linked immunosorbent assay (ELISA) was performed to detect serum levels of IgE,flow cytometry (FCM) to determine the percentage of CD4+IL-9+ T cells,real-time fluorescence-based quantitative PCR (RTFQ-PCR) to measure the mRNA expressions of IL-9 and PU.1 in peripheral blood mononuclear cells (PBMCs).Measurement data were compared between the two groups by independent-samples t test,and relationship among these parameters was assessed by Pearson correlation analysis.Results The expressions of IL-9 and PU.1 mRNAs in patients with chronic urticaria were significantly higher than those in healthy controls (4.44 ± 1.90 vs.3.20 ± 1.78,t =2.60,P＜ 0.05; 3.26 ± 1.59 vs.2.34 ± 1.47,t =2.34,P ＜ 0.05).Compared with the control group,the patient group showed increased percentage of CD4+IL-9+ T cells in peripheral blood (0.63％ ± 0.44％ vs.0.22％ ± 0.12％,t =5.04,P ＜ 0.01) and serum IgE levels ((82.04 ± 31.72) vs.(60.74 ± 28.26) IU/ml,t =2.75,P ＜ 0.01).The percentage of CD4+IL-9+ T cells and levels of IL-9 and PU.1 mRNAs in peripheral blood were all positively correlated with serum levels of IgE in these patients (r =0.596,0.767,0.746,respectively,all P ＜ 0.01).Conclusion T helper type 9 (Th9) cells and IL-9 may take part in the occurrence of chronic urticaria.