A retrospective study of the laboratory results on respiratory specimens received from children under 12 years of age between January 1977 and December 1979 was carried out. These children were either hospital patients, usually with lower respiratory infections, or outpatients on the Influenza Surveillance Programme. The overall virus isolation rate was 26.8%, and the isolation rate among hospital patients, 38.5%. Epidemics or outbreaks were associated with infections due to the influenza viruses, respiratory syncytial virus (RSV), the parainfluenza viruses and the enteroviruses. RSV, parainfluenza virus types 1 and 3, and the adenoviruses caused infection mainly in young children under 3 years of age, while the influenza viruses and Mycoplasma pneumoniae caused infection more frequently in older, school-aged children. There was a strong clinical association of bronchiolitis with RSV and the rhinoviruses, of laryngitis or laryngotracheobronchitis with parainfluenza virus types 1 and 2, of pneumonia with Mycoplasma pneumoniae, and of upper respiratory infection or "flu" with the influenza viruses. The significance of some of these findings is discussed.