Bronchiolitis is the leading cause of pediatric morbidity and healthcare costs. Despite the commonplace use of bronchodilator treatments, like albuterol, in conditions like asthma, their efficacy in bronchiolitis remains controversial due to the heterogeneity in patient response. Although studies indicate that bronchodilators do not enhance outcomes in bronchiolitis, meta-analyses can obscure the heterogeneity of treatment effects. While bronchodilator response genetics have not been explored in bronchiolitis, treatment effectiveness variations often depend on genomic factors. Genome-wide association studies (GWAS) have linked genetic variants with bronchodilator response and outcomes in childhood asthma, suggesting a bronchodilator-responsive genotype. This proposal aims to extend this paradigm to bronchiolitis, addressing the gap in knowledge where GWAS and clinical characteristics intersect. The proposed study's objective is to characterize phenotypic and genotypic variations of children with bronchiolitis and their association with bronchodilator response. We hypothesize that children with bronchiolitis who exhibit clinical and historical characteristics associated with atopy and specific physical findings have genetic variants linked to bronchodilator response. To achieve this, we propose to (Aim 1) define airway responsiveness to bronchodilator treatment in children with bronchiolitis using the change in respiratory score, (Aim 2a) identify the associations between candidate genetic variants and bronchodilator response among children with bronchiolitis, and (Aim 2b) determine the associations between candidate genetic variants and clinical patient data to identify bronchodilator-responsive children with bronchiolitis. A prospective, double-blind, randomized, placebo-controlled trial of a single albuterol dose in children aged 3 to 24 months presenting with bronchiolitis to the emergency department will be conducted to achieve these aims. Patient information and respiratory assessment outcomes will be collected before and after intervention. Blood, urine, DNA buccal swabs, and nasopharyngeal swabs will also be collected. Completion of these aims will result in a novel clinical prediction model for bronchodilator response determination in bronchiolitis, integrating clinical, physical, and genetic data. Furthermore, this research supports the candidates' career development goals of advancing training in clinical trial research design and execution and becoming an expert in clinical and translational methods to enhance pediatric emergency department health and outcomes. Ultimately, this work will inform an R01 application to validate an evidence-based prediction rule for identifying bronchodilator-responsive children with bronchiolitis through a multi-center emergency medicine research network, optimizing therapeutic approaches, and reducing resource use in those with a low likelihood of bronchodilator response.