The transition from intravenous (IY) to oral theophylline in patients hospitalized for asthma frequently is haphazard, with little regard for maintaining therapeutic levels. This study, involving 14 pediatrie patients, investigated several methods of transition from IY to oral, sustained-release theophylline. Our results demonstrated that an oral dose of a theophylline sustained-release product, administered at the time of the last intermittent IV aminophylline dose, provides a smooth transition from IV to oral therapy. Computer analysis demonstrated that when the oral theophylline product was first administered at time intervals of 4 hours or more after the intermittent IV dose, subtherapeutic levels occurred during the conversion period. For patients receiving continuous theophylline infusions, the oral product is best given at the time of discontinuation of the infusion but will be effective if given within 2 hours of the discontinuation of the infusion.