We evaluated the effects of a new semisynthetic macrolide antibiotic, roxithromycin, on the bronchial hyperresponsiveness to histamine in children with asthma. Twelve hospitalized asthmatic children, aged 11 to 15 years (mean age, 12.9 years), were enrolled in this study. They were treated with 150 mg of roxithromycin once a day orally for 8 weeks without any side effects. The PC20 value 4 or 8 weeks after the administration of roxithromycin increased significantly over the initial values (p < 0.05, p < 0.01, respectively). No significant change was observed in serum theophylline concentrations during this study. Serum cortisol level in the morning did not change after the administration of roxithromycin for 4 weeks. These results suggest that administration of roxithromycin may act favorably in the treatment of childhood asthma.