Objective To investigate the percentage of CD4 + CD25 + Treg cells and expression of Foxp3 mRNA in asthmatic patients and the impacts of inhaled steroids.Methods The percentages of CD4 +CD25 + Treg cells was assayed by flow cytometry and the expression of Foxp3 mRNA was detected by RT-PCR in peripheral blood mononuclear cells from the patients with chronic persistent asthma before and after steroids inhalation in comparison with healthy control. The forced expired volumin one second/predicted value( FEV1% pred) and peak expired flow( PEF) were measured by spirometry. Results The level of CD4 + CD25 + Treg cells and the expression of Foxp3 mRNA were lower in asthmatics before steroids treatment than those in control ( P lt; 0. 05) which were increased significantly after steroids treatment ( P lt; 0. 05) .FEV1% pred and PEF were declined significantly than those in control but improved markedly after treatment ( P lt; 0. 05) . Conclusions The insufficiency of amount and function of immue-suppressive CD4 + CD25 +Treg cells may play a role in the pathogenesis of asthma. Inhaled steroids can improve the lung function of asthmatics by upregulating the level of CD4 + CD25 + Treg and Foxp3.