During a study to document the effects of childhood respiratory disease and smoking on the lung function of Sydney schoolchildren, the prevalence of children with asthma was documented. Two groups of children aged 12.6 (6590) and 8.9 years (4893) were studied in schools selected at random from the Sydney metropolitan area. Initially parents filled out a questionnaire recording whether the child had ever been treated for asthma or bronchitis by a physician. The children were seen yearly between 1971 and 1976 and a history of asthma and/or bronchitis present duirng the preceding year -as recorded. The cumulative prevalence of asthma until 1976 was significantly higher in boys than in girls in both age groups. Bronchitis was more prevalent than asthma but the difference between girls and boys was less obvious in the younger age group.