Theophylline is commonly used to decrease airway resistance in patients with bronchopulmonary dysplasia (BPD). Little is known, however, about theophylline's pharmacokinetics in infants' with BPD. The objectives of this study were: (i) to examine the pharmacokinetics of theophylline; and (ii) to assess the intrapatient variation in theophylline serum concentration in BPD. Nineteen patients (gestational age: 27-33 weeks, postnatal age: 12-165 days) received 2-12 mg/kg/day theophylline. Multiple serum concentrations of theophylline were determined in each patient. Apparent clearance of theophylline ranged from 0.28 to 0.51 ml/min/kg. The steady-state trough serum concentration of theophylline ranged from 3.5 to 11.2 micrograms/ml. Based on our data, theophylline doses of 4-7 mg/kg/day may be required to achieve an average steady-state serum concentration of 10 micrograms/ml. Substantial intrapatient (twofold) variability was observed in theophylline concentrations normalized for dose during therapy. The data indicate that the theophylline serum concentration should be monitored for individualizing therapy in infants with BPD.