A diagnosis of 979 respiratory viral infections was made in hospitalized children. Respiratory syncytial virus greatly out-numbered the other viruses: it caused 58% of the total virus infections and occurred in winter epidemics. Influenza A and B virus occurred during late winter and spring, rhinovirus had a seasonal distribution towards spring and autumn, whereas adenovirus types 1, 2 and 5 had no distinct seasonal distribution. Whereas respiratory syncytial virus were mainly associated with bronchiolitis and adenovirus type 7 with pneumonia, rhinovirus infections were most often found in children with episodes of acute bronchial asthma. The influenza A and B and adenovirus types 1, 2 and 5 infections often occurred with extrarespiratory symptoms, especially febrile convulsions.