Bronchiolitis is a potentially severe infection of the airway in infants and children, and among the most frequent diagnoses leading to pediatric intensive care unit admission in infants. This acute infection is caused by an array of viruses, but respiratory syncytial virus (RSV) is the most frequently implicated. The majority of infants hospitalized with bronchiolitis are previously healthy, and half of infants intubated and mechanically ventilated for respiratory failure due to RSV bronchiolitis have no previously identified risk factors. It is likely, therefore, that other factors, particularly genetic heterogeneity of the host, contribute to disease severity. However, no previous study has investigated the association of genetic variants with respiratory failure in children with bronchiolitis. Several categories of candidate genes have emerged as potentially important in the pathogenesis of the disease. Specifically, genetic polymorphisms of surfactants, pattern recognition receptors, receptor adhesion molecules, and cytokines have been examined. The aim is to evaluate these polymorphisms to determine their association with respiratory failure in a cohort of more severely ill children with bronchiolitis.