AbstractFifty‐five children with acute asthma were studied to evaluate the relationship between pulmonary function (improvement in forced expiratory volume in one second, FEVj, and maximum flow at 50% vital capacity, V50) and serum theophylline concentration after intravenous aminophylline.Serum theophylline concentrations and pulmonary function were measured before and after aminophylline therapy.In our study, there was significant improvement in pulmonary function in patients whose baseline serum theophylline concentration was under 5ngper ml, when their post‐infusion levels reached 10 to 15 μg per ml. Significant improvement in pulmonary function was noted in subjects whose baseline FEV1was under 50% of the predicted value when their post‐infusion serum theophylline levels rose to 10 to 15 μg