Although bronchial hyperresponsiveness in asthma is associated with inflammation within the airways, it is not known whether the degree and type of inflammation influence the response to different stimuli and whether pathologic changes of airway structure influence the bronchoconstrictive responses. Therefore, number of inflammatory cells in the epithelium and the lamina propria and the basement membrane thickness were estimated from bronchial biopsies taken in 27 asthmatic subjects (range percent predicted FEV1: 75.6 to 132.1, range of daily PEF variability: 1.9% to 20%) and related to the degree of bronchial responsiveness to ultrasonically nebulized distilled water (UNDW) and methacholine (M). PD20UNDW (provocative dose) was measurable in 15 of 27 patients and ranged between 1.01 and 20.4 ml. PC20M (provocative concentration) ranged between 0.15 and 31.7 mg/ml. In the 15 responders to UNDW, total inflammatory cells (p<0.04) and eosinophils (p<0.015) within the epithelium were higher than in 12 nonresponders to UNDW (PD20 > 34.8 ml). There was no correlation between PD20UNDW and any cell counts whereas negative correlations were found between PC20M and both total inflammatory cells (rs = -0.57; p<0.005) and eosinophils (rs = -0.63; p< 0.0015) within the epithelium. The degree of thickening of subepithelial layer ranged between 7 and 16 micrometers+ (n=26). Thickness correlates both with total inflammatory cells (rs = 0.49; p<0.025) and eosinophils (rs = 0.61; p< 0.003) within the epithelium. Moreover, it was correlated with baseline FEV1 (rs = -0.57; p<0.003) and daily peak expiratory flow (PEF) variability (rs = 0.51; p<0.01). A weak but significant correlation was also found between subepithelial layer thickness and PC20M (rs = -0.42; p<0.04). The results of this study demonstrate that eosinophilic inflammation of bronchial epithelium plays a role in determining UNDW and M responsiveness in asthma. Moreover, they suggest that remodeling of the airways such as thickening of subepithelial layer correlates with indices of asthma severity and could contribute to the degree of M but not to UNDW responsiveness.