McNamara et al described large quantities of IL-9 in the lower airways of infants with severe bronchiolitis and discovered that neutrophils were the source.[10] IL-9 induces mucous cell metaplasia, production of mucus, proinflammatory chemokines and airway hyper-responsiveness.[43]–[45] Genetic linkage studies in humans indicate that IL-9 plays an important role in the pathogenesis of atopic asthma.[46] In earlier studies we were unable to determine if airway concentrations of IL-9 varied with hRSV disease severity because we only examined the bronchoalveolar fluid from ventilated infants. This evidence concerns the gene IL9 and bronchiolitis.