In this double-blind, parallel, single-dose study, bitolterol mesylate aerosol (three sprays, 1.11 mg) and albuterol aerosol (two sprays, 180 mcg) were compared for efficacy of bronchodilation in 120 adolescent and young adult patients with moderate to severe asthma. All patients required regular medications for asthma control. None was steroid dependent. Both medications gave effective bronchodilation within 5 min with maximum effect at 30 to 60 min. Mean percent increase in forced expiratory volume in 1 sec (FEV1) over baseline was higher for bitolterol than for albuterol at all test times up to 8 hr after a dose at which time 20% mean percent increase of FEV1 over baseline was still present in the bitolterol-treated patients. With albuterol mean percent increase in FEV1 fell to 15% over baseline at 5 hr after a dose. Differences in FEV1 increase between the two treatment groups were statistically significant at 4 to 8 hr after a dose. Patients with baseline FEV1 less than 50% of predicted normal had a response to bitolterol that was higher than that observed with albuterol treatment (p less than 0.1). Mean maximum percent increase in FEV1 and median duration of bronchodilation were greater with bitolterol than with albuterol, but the differences were not statistically significant. Both bitolterol aerosol and albuterol aerosol were demonstrated to be safe, effective, and long-acting bronchodilating agents. Both bitolterol and albuterol administered by aerosol had a rapid onset, and the maximum degree of bronchodilation was comparable. However, at the doses studied, bitolterol produced significantly higher increase in FEV1 over baseline at longer times after medication than did albuterol.