We have compared the inflammatory changes in the bronchial mucosa and the increase in the airway hyperresponsiveness in an asthmatic patient during the deterioration of symptoms. A striking increase in the number of bronchial epithelial eosinophils was associated with an increase in both airway hyperresponsiveness and asthma symptoms. During 16 wk of treatment with the inhaled corticosteroid, budesonide, the patient's clinical status and airway hyperresponsiveness improved. This was accompanied by an improvement in the ultrastructure of bronchial mucosa and a decrease in the number of epithelial eosinophils.