A prospective study of respiratory illness history and lung function of 10,898 school children in Sydney was begun in 1971. At the first visit, a history of previous asthma and of bronchitis before and after the first 2 years of life was obtained from a parental questionnaire, and maximal expiratory flow-volume curves were recorded. These lung function studies were repeated yearly between 1972 and 1974, at which time a history of respiratory illness during the previous 12 months and a personal smoking history were recorded, Two groups of children from a random selection of primary and secondary schools in Sydney were studied. Their respective mean ages were 8.9 yr (primary group) and 12.6 yr (high school group) in 1971. Mean values for the maximal flow at 50 percent of the forced vital capacity were lower in children with a history of bronchitis and/or asthma than those in the control group. This was true in both age groups in both sexes. No differences were found in the 0.5-s forced expiratory volume or forced vital capacity. The differences in maximal flow at 50 per cent of the forced vital capacity were present again in 1974. In 1974, the data from smokers were compared with those from nonsmokers; small differences were found. The results suggest that bronchitis in infancy and childhood as well as asthma may affect lung function as children grow, that the abnormality may not be detected by the forced expiratory volume, that the abnormality persists, and that it is possible that the abnormality is further affected by smoking.