Serum theophylline levels produced in asthmatic children by sustained-release theophylline tablets (TheoDur) were studied. Nineteen patients received for 14 days a dose of 5.5 to 13.1 mg/kg (mean 9.1 mg/kg) of amhydrous theophylline (as sustained-release TheoDur tablets) every 12 hours. Theophylline serum levels were assayed, by cation-exchange high-performance liquid chromatography, immediately before and at 4, 6 and 10 hours after the first day-five dose. Symptoms of asthma and theophylline toxicity were recorded. Mean peak-trough difference for the 6-to-10-years age group (4.5 +/- 1.6 microgram/ml) was not significantly different than that of the 11-to-17-year age group (5.2 +/- 3.2 micrograms/ml) (p greater than 0.1). Therapeutic serum theophylline levels (8 to 20 micrograms/ml) were maintained throughout a 12-hour period in 12 patients. Two patients had side effects possibly attributable to theophylline. Four patients reported asthamtic symptoms on two or more evenings; none required emergency treatment. The study suggests that sustained-release theophylline tablets administered every 12 hours can maintain therapeutic serum levels in children.