Previously published data of plasma concentrations and pharmacologic effects obtained after rapid intravenous injection and during constant rate intravenous infusion of theophylline in asthmatic children were analyzed pharmacokinetically to determine the relationship between effect on pulmonary function (improvement of forced expiratory volume in the first second) and drug level. The pharmacokinetics of distribution and elimination of theophylline are well described by a linear two-compartment open model consisting of a central and a tissue compartment. There is an essentially linear correlation between the intensity of effect and the logarithm of the amount of drug in the tissue compartment. This correlation exists both with decreasing and increasing drug concentrations, i.e., after intravenous injection and during intravenous infusion of theophylline.