• The benefit of β-adrenergic agonists in the treatment of acutely wheezing infants and young children has not been well documented in the outpatient setting. To determine the efficacy of nebulized metaproterenol sulfate, 74 children aged 36 months or younger with acute wheezing participated in a double-masked, randomized, placebo-controlled clinical trial. Children received nebulized metaproterenol, either as an initial treatment or after a control treatment with normal saline solution. At baseline and 20 minutes after each treatment, an assessment was made that included measurements of heart rate, respiratory rate, oxygen saturation, and clinical variables related to respiratory compromise with the use of a standardized respiratory distress index (RDI). Children who received saline solution as initial therapy had no significant differences from baseline in any of the assessment measures. After metaproterenol therapy, children demonstrated an increase in heart rate ([mean±SD] 147±14 beats per minute vs 153±16 beats per minute), a decrease in respirations (50/min±5/min vs 45/min±7/min), improvement (lower scores) on the RDI (24±4 vs 15±2), and an increase in oxygen saturation (94.1%±2.7% vs 95.3%±3.0%). Patients aged 12 months or younger (n = 37) benefited from metaproterenol treatment (improvement in respiratory rate and RDI) but not to the same degree as children aged 24 months or older (n = 23) (improvement in respiratory rate, RDI, and oxygen saturation). Compared with assessments made before metaproterenol treatment, patients with respiratory syncytial virus infection (n = 21) had improvement in respirations (52/min±7/min vs45/min±6/min) and RDI scores (22±4 vs 14±3). Based on a priori criteria (reduction in a premedication respiratory rate of 20% and an RDI score of 50%), responders to metaproterenol therapy included 45% of the entire sample and, respectively, 40% of those aged 12 months or younger, 52% of those aged 24 months or older, and 48% of patients who tested positive for respiratory syncytial virus. Although there appears to be an age-dependent degree of response, metaproterenol is effective in relieving the respiratory distress of young acutely wheezing children, including those with documented respiratory syncytial virus bronchiolitis.