Objective: To investigate the cytokine response pattern (IL 4/IFN γ ) in infants with RSV infections and bronchiolitis during the acute phase. Methods: Four color flow cytometry was used to measure intracellular IL 4 and IFN γ expressions in peripheral blood CD3+ and CD8+ lymphocytes from RSV infected and bronchiolitis infants. Serum IL 4 and IFN γ levels were also determined. Results: RSV infected and bronchiolitis infants showed no statistical differences from not RSV infected or pneumonia infants and control in the frequency of IL 4 and IFN γ expressions in CD3+CD8 lymphocytes, showed no obvious Th1/Th2 imbalance, while IFN γ was expressed much more frequently in CD3+CD8+ lymphocytes. Systematically, RSV infected and bronchiolitis infants showed much lower levels of serum IL 4 and IL 4/IFN γ ratios and much higher serum IFN γ levels than control. However, there were no statistical differences in the above three indices between RSV infected and not RSV infected infants or between bronchiolitis and pneumonia infants, except that bronchiolitis infants had a higher level of serum IFN γ than pneumonia infants statistically. Conclusions: There is no type 2 cytokine response predominance in the acute phase of RSV infection and bronchiolitis. IL 4 production is suppressed and IFN γ production upregulated, the latter being most prominent in bronchiolitis infants. Objective: To investigate the cytokine response pattern (IL 4 / IFN γ) in infants with RSV infections and bronchiolitis during the acute phase. Methods: Four color flow cytometry was used to measure intracellular IL 4 and IFN γ expressions in peripheral blood CD3 + and CD8 + Results: RSV infected and bronchiolitis infants showed no statistical differences from not RSV infected or pneumonia infants and control in the frequency of IL 4 and IFN γ expressions in Systematically, RSV infected and bronchiolitis infants showed much lower levels of serum IL4 and IL4 / IFN γ ratios and CD3 + CD8 lymphocytes, showing no obvious Th1 / Th2 imbalance while IFNγ was expressed much more frequently in CD3 + CD8 + lymphocytes. However, there are no statistical differences in the above three indices b etween RSV infected and not RSV infected infants or between bronchiolitis and pneumonia infants, except that bronchiolitis infants had a higher level of serum IFN γ than pneumonia infants statistically. Conclusions: There is no type 2 cytokine response predominance in the acute phase of RSV infection and IL 4 production is suppressed and IFN γ production upregulated, the latter being most prominent in bronchiolitis infants.