The bronchodilator efficacy of oral aminophylline and aerosol metaproterenol was compared in 18 asthmatic patients in a stable clinical condition. Treatment consisted of four regimens in a double-blind random sequence on four different days after withholding bronchodilators: (1) the administration of aminophylline tablets, 0.4 to 0.6 g, orally, (2) 3 puffs of aerosol metaproterenol administered in a sequential manner, (3) a combination of both, (4) placebos. Both oral aminophylline and aerosol metaproterenol produced significant bronchodilatation measured by forced expiratory volume in 1 second (FEV1). After the administration of aerosol metaproterenol, there was a more prompt and larger improvement in FEV1 than after the administration of aminophylline (p less than 0.01). The combined therapy produced a response which was larger, but not significantly, than the effect of metaproterenol. Side effects were frequent after the administration of aminophylline but absent after aerosol metaproterenol. The advantages of the aerosol adrenergic agonists are the prompt onset of action and efficacy, small dosage preferentially delivered to the bronchial tree and lack of side effects.