It has been demonstrated that bronchial hyperresponsiveness is a characteristic feature of bronchial asthma, and airway inflammation plays an important role in bronchial hyperresponsiveness. Erythromycin is an antibiotic extensively used worldwide which is also reported to have anti-inflammatory action. This study was designed to clarify whether erythromycin could favorably alter bronchial responsiveness in patients with bronchial asthma. To estimate bronchial responsiveness, histamine challenge was performed in 23 patients with bronchial asthma (atopic type, 11; nonatopic type, 12). All patients were treated for ten weeks with erythromycin, 200 mg three times daily, orally. After ten weeks' treatment, PC20, an index of bronchial sensitivity, was increased significantly. There was no difference between atopic and nonatopic patients in the improvement of PC20. It was concluded that erythromycin reduces the severity of bronchial responsiveness in patients with bronchial asthma.