To determine whether there is an airway IFN response in infants with acute bronchiolitis and to establish whether the rate of such a response is related to the severity of illness, the expression of some IFN-induced genes was measured in nasopharyngeal washes from 39 infants with acute bronchiolitis. The results indicate that in infants with a virus-associated acute bronchiolitis there is a strong activation of IFN system and that the severity of illness is inversely related to the level of expression of IFN-induced genes. This suggests that the IFN response plays an important role in determining virus-associated respiratory disease in early life.