We performed 211 lung function measurements on 93 children in the first year after they had been admitted with acute bronchiolitis. During the convalescent phase of the illness, 77% of the infants were hyperinflated with a thoracic gas volume greater than 40 ml/kg and 3 months later 43% were hyperinflated. Twelve months after the initial illness, 17% still had lung function abnormalities and most of these children have had lower respiratory tract symptoms. For the group as a whole about 60% have had at least one episode of wheezing. Specific conductances were significantly lower in children from atopic families, indicating worse lung function, but the significance of this finding is unclear.