The lung function of 11,497 Sydney schoolchildren was measured at regular intervals over a ten year period. The prevalences of respiratory illness in infancy and childhood and cigarette smoking in teenage years were documented and the effects on lung function assessed. Using maximal expiratory flow at 50% of forced vital capacity (V 50), small but persisting changes in lung function were found in both males and females who had had bronchitis and/or asthma before the age of 2 years. Subsequent bronchitis had an additional effect on lung function in these children with early respiratory illness. Subsequent asthma had the greatest effect on lung function and the deficit increased as these children approached adult life. Deficits in lung function attributable to smoking were found in some children by the age of 14 years and occurred in children with previously normal lung function. Smoking in children who had a history of asthma was associated with severe abnormalities of lung function.