A double-blind randomized study of 50 patients was conducted to evaluate the efficacy and safety of terbutaline sulfate aerosol and isoproterenol sulfate aerosol in the treatment of patients with moderate to severe asthma. The patients were randomized to receive either terbutaline or isoproterenol (two sprays, four times a day) for 14 weeks with office evaluations at three week intervals. On that day pulmonary function studies as well as electrocardiographs (EKG), blood chemistry analysis and urinalysis were performed before, and pulmonary function studies and EKG at intervals for four hours after, study medications were taken. Terbutaline sulfate was shown to have a much more sustained effect and a larger "area under the curve" than isoproterenol, but the magnitude of the peak effect was similar. There was no statistically significant difference in pulmonary function responsiveness throughout the duration of this study with either medication. Few patients experienced serious adverse reactions and no EKG changes were noted.