This study of 183 ambulatory patients with steroid-dependent asthma was conducted to evaluate the efficacy and safety of nebulized bitolterol mesylate solution (0.2%) compared to isoproterenol hydrochloride solution (0.3%). A double-blind, randomized, parallel-group, repetitive-dose design was followed at nine centers for 3 months. Patients received either 1.0 mg of bitolterol or 1.5 mg of isoproterenol three times a day with a closed, intermittent-flow nebulization system. Pulmonary function was evaluated on four 8-hour office visits at approximately 30-day intervals. Efficacy was based on a 15% increase in FEV1 over baseline. Both medications resulted in bronchodilatation within 5 minutes, whereas nebulized bitolterol was statistically superior (p less than 0.05) to nebulized isoproterenol in terms of duration of action and area under the curve. The mean FEV1 response to bitolterol therapy remained greater than 15% over baseline for 5 to 8 hours on the four test days compared to 2 to 4.75 hours for isoproterenol therapy. Both medications were well tolerated. Adverse reactions were transient, and most were mild to moderate. Tremor was the most frequent side effect occurring in approximately 30% of the patients in both groups. There were no clinically significant laboratory changes or electrocardiographic findings. Nebulized bitolterol mesylate was found to be a safe and effective bronchodilator in steroid-dependent patients with asthma.