Procaterol hydrochloride aerosol, a potent beta 2-adrenergic bronchodilator, was evaluated in a double-blind, placebo-controlled study for efficacy and safety in 210 patients with documented mild to moderate reversible airway obstruction. Patients were randomized to receive procaterol in two inhalations (high dose) or one inhalation (low dose), 0.01 mg/inhalation, three times daily, or placebo. Pulmonary function tests were recorded at five and 30 minutes and hourly for eight hours after the first dose and following 1 and 2 weeks of treatment. Both doses of procaterol produced significantly greater improvement in PFTs at one hour and for up to seven hours after dosing compared with placebo (p less than 0.05). Mean percent increases in FEV1 were 35% in the high-dose group and 29% in the low-dose group at week 2. The high-dose group showed no loss of duration of bronchodilation with continued dosing. Improvement in PFTs and peak flow rates was significantly greater in the high-dose than in the low-dose group (p less than 0.05). Tremor was the most frequent side effect. Procaterol had no effect on electrocardiograms, heart rate, blood pressure, or clinical laboratory tests. The high dose of procaterol aerosol was shown to be an effective and well-tolerated bronchodilator with a rapid onset and long duration of action.