• Twenty patients with asthma (mean age, 10.9 ±2 years) entered a six-week, randomized, double-blind, crossover comparison of sustained-release (S-R) terbutaline sulfate (Bricanyl Durules) vs S-R theophylline (Theo-Dur). In each two-week study period each patient received S-R theophylline twice daily in doses previously adjusted to give serum theophylline concentrations in the range of 10 to 20 mg/L (56 to 111 μmol/L); or S-R terbutaline sulfate, 5 mg twice daily; or S-R terbutaline sulfate, 7.5 mg twice daily. All treatment regimens produced significant improvement in one or more pulmonary function test values compared with prestudy values. The incidence of acute asthma episodes were similar during each treatment regimen. No clinically significant differences occurred between the regimens for daily symptom scores, peak expiratory flow rates, or use of a terbutaline metereddose inhaler. At the end of the theophylline treatment period, the mean (±SD) theophylline level 12 to 14 hours after the last dose was 10.1± 3.3 mg/L (56±18 μmol/L); at the end of the terbutaline treatment periods, the mean trough terbutaline levels were 2.22 μg/L (9.9±4.4 nmol/L) (S-R terbutaline sulfate, 5 mg twice daily) and 3.07 μg/L (13.7± 5.4 nmol/L) (S-R terbutaline sulfate, 7.5 mg twice daily). Adverse effects, including tremor, occurred with similar frequency during all three drug regimens. Sustained-release formulations of theophylline and terbutaline, in the dosages studied, provided comparable control of asthma symptoms.