Although theophylline has long been the primary drug used for the management of reversible obstructive airway disease, current recommendations for determining theophylline dosage have not been dependable and, when applied to individual patients, have resulted in a wide range of serum theophylline concentrations (STC) and frequent adverse reactions.1–3The objective of this study was to evaluate the clinical reliability of using theophylline clearance values to adjust the theophylline infusion rate to achieve a desired STC and to determine the minimum time required for administration of a continuous theophylline infusion in a given population of patients before obtaining a single STC for the clinically reliable calculation of theophylline clearance.