In a series of 54 children under 1 year old with respiratory syncytial virus infection, the highest proportion of severe illnesses (bronchiolitis and pneumonia), and the highest proportion of `slow' antibody responses, were found in children under 6 months. Severe illness may therefore be related to immunological immaturity and to lack of protection by maternal antibody, rather than to an allergic response as has been suggested. A high male to female ratio (approximately 2: 1) was found both in the present series, and in two larger series of respiratory syncytial virus infection in infants.