Our study was designed to assess potential cardiovascular adverse effects in clinically stable asthmatic children due either to oral sustained-release theophylline or theophylline in combination with an inhaled beta-2 adrenergic agonist. Twenty-five asthmatic children were evaluated while receiving no drugs, theophylline alone, and theophylline with an inhaled beta-2 adrenergic agonist. In each phase all patients underwent 24- to 48-hour Holter monitoring and a maximal treadmill exercise test. The results show that neither theophylline alone nor combined therapy was associated with any relevant cardiovascular adverse effect, including ectopic cardiac activity. A nonsignificant increase in mean heart rate was observed between each period of study. The data suggest that the use of theophylline either alone or in combination with a beta-2 adrenergic agonist in clinically stable asthmatic children is not associated with any serious cardiovascular effect.