INTRODUCTION: Our previous BAL study demonstrated that there were different implications of cell types between acute bronchiolitis and acute asthma. No data on airway cytokine responses in these two diseases is available. The aim of this study was to compare the pattern of cytokine response (Th1 or Th2) in the airways between these two diseases. MATERIALS AND METHOD: Using BAL, this study investigated the cytokine levels of BALF in children with acute asthma (n=12, median age 5.0 years) and infants with acute bronchiolitis caused by respiratory syncytial virus (RSV) (n=14, median age 1.4 years). Comparisons were made with normal controls (n=8, median age 5.1 years). BAL cell counts and differentials were determined. Interferon(IFN)-gamma (Th1), interleukin(IL)-4, and IL-5 (Th2) levels were measured in concentrated BALF by ELISA. RESULTS: The IL-5 levels were significantly increased in asthma group (p=0.000) and bronchiolitis group (p=0.023) compared with control group. The IL-5/IFN-gamma ratios were signifi- cantly increased in asthma group (p=0.000) and bronchiolitis group (p=0.039) compared with control group. The asthma group had significant elevations in IL-5 levels (p=0.001) and IL-5/ IFN-gamma ratios (p=0.046) compared to bronchiolitis group. IL-4 levels were at or below the level of detection in all subjects. The number of BAL eosinophils correlated (r=0.71, p=0.002) with the levels of BALF IL-5 in asthma group and bronchiolitis group. CONCLUSION: These findings suggest that RSV bronchiolitis results in Th2-type response as asthma, which could provide a valuable framework for explaining a link between RSV bron- chiolitis and asthma.