Inflammation of the airway wall and airway hyperresponsiveness are consistent features of chronic asthma. We investigated how damage of the bronchial epithelium is related to airway hyperresponsiveness and how bronchial infiltration of eosinophils and lymphocytes is related to bronchial epithelial damage. We examined the biopsy specimens of bronchial mucosa taken from 19 patients with chronic asthma by electron microscopy. We also measured the incidence of opening of epithelial tight junctions, the widening of intercellular spaces in the epithelium, and the density of infiltrated eosinophils and lymphocytes in bronchial mucosa. Airway responsiveness was accessed by measuring PC20-acetylcholine (PC20-ACh). The inflammatory cells in the airway mucosa were counted by electron microscopy. Lymphocytes were most abundant, being 54.5% of the cells counted; eosinophils were 22.1%, neutrophils were 4.9%, and mast cells were 4.6%. A significant correlation was noted between the density of eosinophils and that of lymphocytes infiltrated in the airway mucosa (r = 0.80, p less than 0.01), suggesting that T cells may potentiate eosinophil infiltration. With increased density of eosinophils infiltrated in bronchial mucosa, both the incidence of opening of tight junctions of epithelial ciliary cells and the degree of widening of intercellular spaces in epithelium increased significantly (r = 0.51, p less than 0.05; r = 0.52, p less than 0.05), suggesting that eosinophils are related to damage of the bronchial epithelium. No correlation was observed between the density of lymphocyte infiltration and the degree of epithelial damage.(ABSTRACT TRUNCATED AT 250 WORDS)