Objective: To describe the pattern of alterations in creatine phosphokinase (CPK) and creatine phosphokinase-myocardial isoenzyme (CPK-mb) levels and electrocardiogram (ECG) changes in children treated with intravenous infusion of isoproterenol, as determined by elevation of CPK-mb levels/indices and/or ischemic ECG changes. Design: Prospective observational study. Setting: A pediatric intensive care unit of a university teaching hospital. Subjects: Twenty-two critically ill children (age: 3 months-17 years) with severe reactive airway disease who required intravenous isoproterenol. Interventions: None. Measurements and Main Results: Twenty-two children with severe reactive airway disease [pH 7.25 (7.01-7.54), Paco2 55 (23-113), and Pao2 92 (54-361)] were enrolled in the study. Baseline ECGs prior to intravenous isoproterenol were unremarkable. Baseline CPK-mb levels/indices were found to be abnormally elevated in 5 children prior to intravenous isoproterenol. Four of these children manifested a decrease in CPK-mb levels/indices while on isoproterenol infusion. Seventeen of 22 children had normal CPK-mb levels/indices prior to isoproterenol infusion. Only 4 of these children experienced abnormal elevations during intravenous isoproterenol. CPK-mb levels/indices returned to normal in all children within 6 h of discontinuation of intravenous isoproterenol. No dysrhythmias or deaths were noted in patients who received intravenous isoproterenol. Conclusion: Intravenous isoproterenol is an appropriate therapeutic modality for children with severe reactive airway disease unresponsive to conventional therapy.