OBJECTIVE: To study the roles of CD4(+)CD25(+) regulatory T cells and Foxp3 mRNA in peripheral blood as well as serum total immunoglobulin E (IgE) in the pathogenesis of bronchiolitis caused by respiratory syncytial virus (RSV). METHODS: The proportion of CD4(+)CD25(+) regulatory T cells and expression of Foxp3 mRNA in peripheral blood, and total serum IgE level were tested by flow cytometry, RT-PCR and ELISA respectively in 57 children with RSV bronchiolitis (26 atopic patients and 31 nonatopic patients). Twenty five healthy children were used as the control group. RESULTS: The proportion of CD4(+)CD25(+) regulatory T cells in peripheral blood in children with bronchiolitis, either in the atopic (7.7+/- 1.6%)or the nonatopic group (8.8+/- 2.1%), was significantly lower than that in the control group (10.5+/- 1.6%) (P< 0.01). Foxp3 mRNA expression in peripheral blood was significantly lower in both atopic and nonatopic children with bronchiolitis than that in the control group (P< 0.01). Significantly increased total serum IgE level was noted in both atopic (241.2+/- 102.5 IU/mL) and nonatopic children (125.5+/- 63.2 IU/mL) with bronchiolitis compared with that in the control group (27.2+/- 10.5 IU/ml) (P< 0.01). There were significant differences in the proportion of CD4(+)CD25(+) regulatory T cells and Foxp3 mRNA expression in peripheral blood (P< 0.05) as well as total serum IgE level (P< 0.01) between the atopic and the nonatopic group. The proportion of CD4(+)CD25(+) regulatory T cells (r=-0.70, P< 0.01) and Foxp3 mRNA expression in peripheral blood (r=-0.79, P< 0.01) were closely negatively correlated to total serum IgE level. CONCLUSIONS: Both the proportion of CD4(+)CD25(+) regulatory T cells and Foxp3 mRNA expression in peripheral blood were reduced, in contrast, the total serum IgE level increased in children with RSV bronchiolitis. This suggested that CD4(+)CD25(+) regulatory T cells and Foxp3 mRNA together with IgE participated in the pathogenesis of RSV bronchiolitis.