Background Recent studies have shown that T helper type-2 (Th2) cells can induce the apoptosis of CD4+CD25+ Treg cells or resist the immunosuppressive effect of Treg cells.We hypothesize that an imbalance of Th2/Treg is present in patients with allergic asthma.Methods Twenty-two patients with mild asthma,17 patients with moderate to severe asthma,and 20 healthy donors were enrolled.All patients were allergic to house dust mites.The proportion of peripheral blood CD4+CD25+ Treg cells and Th2 cells were determined by flow cytometry.The concentration of interleukin (IL)-10,transforming growth factor (TGF)-β and IL-4 in plasma was determined by enzyme linked immunosorbent assay.In these subjects,peripheral blood mononuclear cells from 17 mild asthmatic patients,13 moderate to severe asthmatic patients and 14 healthy donors were acquired and expression of forkhead box P3 (Foxp3) and GATA-3 mRNA was detected by reverse-transcriptase polymerase chain reaction.Results Compared with healthy donors and patients with mild asthma,the percent of CD4+CD25+ Treg cells and plasma IL-10 levels were decreased in patients with moderate to severe asthma.There were no significant differences in Foxp3 mRNA expression among three groups,but a downward trend seen among patients with asthma.However,the percent of Th2 cells,IL-4 levels and expression of GATA-3 mRNA was markedly higher in patients with mild and moderate to severe asthma than in the control group.The ratio of Th2/Treg and their cytokines was increased in allergic asthma,especially for moderate to severe asthma.The ratio of GATA-3/Foxp3 mRNA was also increased in allergic asthma.In patients with moderate to severe asthma,the percentage of peripheral blood Treg cells was negatively correlated to the percentage of Th2 cells and IL-4 levels.Conclusions The decline of CD4+CD25+ Treg cells in patients with moderate to severe asthma may play an important role in progress of the disease.Furthermore,the deficiency of CD4+CD25+ Treg cells was associated with the overexpression of Th2 response.