Theophylline has long been a cornerstone of bronchodilator therapy for asthma and chronic obstructive lung disease, but until recently its use carried the risk of serious, even fatal, toxicity. With recent advances in understanding of this drug's pharmacologic properties and pharmacokinetic characteristics, serious toxicity now can usually be avoided. The dosage requirements for theophylline demonstrate wide interpatient and intrapatient variation. Several factors responsible for this variation, most notably age, have been identified and used in definition of dosage guidelines for different patient groups. However, substantial variation still exists within these groups, necessitating measurement of serum concentration in selected patients. The measurements obtained are used to titrate the patient's dosage requirement in such a way as to maximize therapeutic response while minimizing risk of toxicity.