Respiratory syncytial virus (RSV) is the most important cause of lower respiratory tract infection in infants and young children. Around 20 000 RSV-infected infants require hospitalization in the UK during each yearly epidemic, which is about 3% of the birth cohort. Most children are infected by 2 years of age. Risk factors for severe disease include young infants, prematurity, chronic lung and cardiac conditions or immunodeficiency. Humoral immunity is incomplete and short-lived, yet reinfections cause less severe disease. RSV infects infants despite the presence of specific neutralizing antibodies. RSV infection can be linked to the development of individual wheezing episodes. A competent cellular immune system is necessary to reduce disease severity. RSV infection provokes an RSV-specific T-lymphocyte response with the release of cytokines. There is a delicate balance between the protective and disease-enhancing effects of the host's immune response to RSV infection.