Of the 180 children admitted to hospitals in Tyneside in the first year of life with proved respiratory syncytial virus lower respiratory tract infection, 130 were seen for review 10 years later and 34 of the remaining 50 children accounted for. Skin tests, lung function tests, and histamine-challenge and exercise tests for bronchial lability were undertaken in over 100 of the index children and a similar number of control children. A total of 55 (42%) of the 130 index children had had further episodes of wheeze, while only 21 (19%) out of 111 controls had ever wheezed; but few (6.2% v 4.5%) had troublesome symptoms at the age of 10. There was a threefold increase in the incidence of bronchial lability in the index children but no excess of atopy. Maximum expiratory air flow was reduced throughout the vital capacity manoeuvre in the index children, even when those with a history of recurrent wheeze were excluded. Results of single-breath nitrogen washout tests were normal, however, suggesting that ventilation was not appreciably uneven, even though expiratory flow was restricted. These differences might have been caused by infection damaging the growing lung but might also be explained by pre-existing differences in the airway, rendering certain children more susceptible to symptomatic infection when first challenged by the virus in infancy.