Theophylline level variability obtained with standard slow-release theophylline tablets was compared to that obtained with a new sustained-release theophylline suspension in asthmatic children. The sustained-release preparations were administered every 12 hours and serum samples were collected after ten days of treatment during the steady-state period. In this crossover study, the sequence of the two regimens (slow-release tablets versus slow-release suspension) was selected at random. Both preparations of theophylline were administered at a mean dosage (+/- s.d.) of 19.3 +/- 1 mg/kg/day. The results of our study demonstrated that theophylline serum levels remain within the therapeutic range for both preparations, but there was a greater fluctuation with theophylline suspension compared to the tablets. The tolerability of the two drugs was satisfactory.