OBJECTIVE: To explore the pathogenesis of respiratory syncytial viruses (RSV) bronchiolitis and its treatment with glucocorticoids. METHODS: The pediatric patients with RSV bronchiolitis were divided into an atopic group (n = 50) and a non-atopic group (n = 50) based on whether there were IgE elevation, eczema and dermatitis. Another 25 normal subjects were chosen as a control group. Divided into mild, medium and severe groups, they were finally randomly divided into hormone (dexamethasone) and non-hormone groups. The proportion of CD4+ CD25+ regulatory T cells (Treg) and the expression of Foxp3 mRNA were tested by flow cytometry and RT-PCR retrospectively. RESULTS: The proportion of CD4+ CD25+ Treg and the Foxp3 mRNA expression in the control, non-atopic, and atopic groups reached (10.5 +/- 1.6)% and 0.34 +/- 0.11, (8.8 +/- 2.2)% and 0.26 +/- 0.08, (7.6 +/- 1.8)% and 0.21 +/- 0.09, respectively. There were significant differences among these groups (all P < 0.05). The mild, medium and severe bronchiolitis groups reached (9.7 +/- 1.6)% and 0.28 +/- 0.08, (7.8 +/- 2.1)% and 0.24 +/- 0.06, (6.7 +/- 1.3)% and 0.20 +/- 0.07 respectively (all P < 0.05). The proportion of CD4+ CD25+ Treg and the expression of Foxp3 showed significantly negative correlations with severity (r = -0.62, -0.71, both P < 0.01). That is, they correlated with the severity of disease. The proportion of the CD4+ CD25+ Treg and the expression of Foxp3 mRNA of the hormone group were higher than those of the non-hormone group [(9.5 +/- 2.1)% and 0.33 +/- 0.10 vs (8.5 +/- 1.8)% and 0.27 +/- 0.12, P < 0.05 and < 0.01] respectively. CONCLUSION: CD4+ CD25+ Treg and Foxp3 mRNA are involved in the inflammation of bronchiolitis. And the levels of CD4+ CD25+ Treg and Foxp3 mRNA level is an objective indicator of the severity of RSV bronchiolitis. The effect of glucocorticoids upon RSV bronchiolitis may be in part due to the direct enhancement of production and functions of CD4+ CD25+ Treg and Foxp3 mRNA.