The pathogenesis, epidemiology, clinical features, sequelae, and treatment of bronchiolitis are reviewed. Acute bronchiolitis is the most common severe lower-respiratory-tract infection of infancy. During epidemics, more than 80% of cases may be caused by respiratory syncytial virus (RSV). Although signs and symptoms may become severe, most infections are self-limited and improvement occurs within several days. Approximately 1-2% of infants less than one year of age require hospitalization. Generally, patients who develop severe, life-threatening RSV bronchiolitis are those with underlying cardiopulmonary disease, immunosuppression, bronchopulmonary dysplasia, or a history of premature birth. In severe bronchiolitis, necrosis of the respiratory epithelium, excessive mucus production, and lymphocytic infiltration result in edema, dense plugs of debris, and subsequent bronchiolar obstruction. IgE-mediated reactions and release of inflammatory mediators may result in exacerbation of acute obstruction and may contribute to chronic obstructive pulmonary dysfunction, a common sequela of bronchiolitis. Patients hospitalized with bronchiolitis usually require supportive therapy and may require mechanical ventilation. Based on recent data, a trial of aerosolized beta 2 agonists is warranted in all patients. Systemic corticosteroids have not proved efficacious and have a limited role in the treatment of acute bronchiolitis. Inhaled corticosteroids may be useful in reducing the severity of chronic wheezing that may follow acute bronchiolitis. Ribavirin may be considered in patients with severe illness or in those at high risk for severe RSV disease. Intravenous immune globulin may have a role in the treatment of lower-respiratory-tract infections involving RSV; however, since few studies have been performed in humans, it is not possible to determine its place in the treatment of bronchiolitis. A trial of aerosolized beta 2 agonists is warranted in patients with bronchiolitis. Ribavirin may be considered in patients with severe disease or those at high risk for severe disease.