Objective To explore the role of CD4+CD25+ Treg cells in chronic pulmonary infection caused by Pseudomonas aeruginosa(PA).Methods Sixty SD rats were randomly divided into a PA group and a control group(n=30 in each group).Chronic lung infection model was established by implantation of silicone tube precoated with PA into the main bronchus.Twenty-eight days later Treg cells in peripheral blood were measured by fluorescence-activated cell sorting（FACS）.Levels of IL-10 and TGF-β in serum were assayed by ELISA.The expression of Foxp3 mRNA in spleen was measured by RT-PCR.Pathological changes of lung tissue were studed by HE staining.Results Treg/CD4+ T cells in the PA group were significantly more than those in the control group[（19.79±6.45）% vs （5.15±0.47）%，Plt;0.05].The levels of IL-10 and TGF-β were (231.52±54.48)pg/mL and （121.05±7.98）pg/mL in the PA group respectively，which were significantly higher than those in the control group[（35.43±23.56）pg/mL and （36.02±8.94）pg/mL].The expression of Foxp3 mRNA in the PA group was significantly higher compared with the control group(0.80±0.044 vs 0.25±0.054，Plt;0.05）.HE staining revealed that PA caused a intensive inflammatory reaction with lymphocytes infiltration.Conclusion CD4+CD25+ Treg cell is up-regulated and plays an important role in chronic lung infection caused by Pseudomonas aeruginosa.