The efficacy and side effects of oral or inhaled terbutaline were examined in 13 mild-to-moderate asthmatic patients (ages 12 to 71 years) on maintenance theophylline (levels 10 to 25 micrograms/ml). In a crossover design, each patient received for two weeks oral terbutaline, 5 mg qid, inhaled terbutaline, 400 micrograms qid, or identical placebo tablets or metered-dose inhalers. Prior to each double-blind period, terbutaline was given for two weeks to ensure development of beta-adrenergic subsensitivity. Home-monitored peak flows and need for supplementary bronchodilators were significantly improved with both oral and inhaled terbutaline, but only the inhaled drug significantly decreased symptoms. Long-term terbutaline, either oral or inhaled, added to maintenance theophylline improved the patients' pulmonary function. The inhaled drug also improved the patients' clinical status as reflected by asthma symptom scores and need for extra bronchodilator. Combining terbutaline with theophylline did not produce evidence of cardiotoxicity.