Respiratory syncytial virus (RSV) infection has been implicated in the pathogenesis of bronchial asthma. In both diseases, interleukin (IL)-4 and IL-13 play important roles. By investigating IL4 and IL13 polymorphisms in 131 children with severe RSV infection and 270 control subjects, we found an association between IL13 polymorphism -1112C/T and severe RSV infection (P = .026). Furthermore, certain haplotypes showed an even stronger association with severe RSV infection (P = .0008). The results suggest that there is a common genetic background in children with severe RSV infection and bronchial asthma. More studies are needed to clarify whether RSV infection provokes asthma or whether RSV infection occurs in children who are genetically predisposed to a pronounced T helper 2 immune response and subsequently develop bronchial asthma.