We have undertaken detailed cellular and ultrastructural examination of bronchial biopsies and bronchial lavage fluid from allergic asthmatic patients in order to determine the nature and degree of the inflammatory processes in mild allergic asthma. Eight atopic asthmatic patients (mean PC20 histamine, 0.90 mg/ml) and four nonasthmatic control subjects underwent fiberoptic bronchoscopy. All asthmatic subjects were clinically stable for 2 wk prior to bronchoscopy and required either no treatment or inhaled albuterol alone. A single 50-ml bronchial wash was undertaken, followed by endobronchial biopsy of subcarinae. These procedures were repeated in the asthmatic subjects 18 h after bronchial provocation with allergen or methacholine. Subsequently, all subjects underwent bronchial reactivity testing with inhaled histamine. The clinical and physiologic data were not revealed to the pathologist interpreting the specimens. The asthmatic subjects shed a significantly greater number of epithelial cells into the lavage fluid than did the nonasthmatic subjects (7.23 versus 1.48 × 104/ml, p = 0.048). There was a statistically significant inverse correlation between the lavage epithelial cell count and bronchial reactivity (rho = −0.64, p = 0.03). In the asthmatic subjects, but not in the control subjects, there was extensive deposition of collagen beneath the epithelial basement membrane, mast cell degranulation, and mucosal infiltration by eosinophils, which exhibited morphologic evidence of activation. Eosinophils, monocytes, and platelets were found in contact with the vascular endothelium, with emigration of eosinophils and monocytes in the asthmatic subjects. These changes were found irrespective of bronchial challenge with allergen. We conclude that allergic asthma is accompanied by extensive inflammatory changes in the airways, even in mild clinical and subclinical disease.