OBJECTIVE To determine the burden of asthma in children. METHODS A cross sectional, randomised, computer assisted telephone survey of a community based sample of 2020 children aged 5-14 years in western Sydney (New South Wales) over a 20 day period from June 2000 to July 2000. RESULTS Main outcome measures were carer reported history of asthma diagnosis, hospital presentation/admission for asthma, recent use of anti-asthma medications, and recent respiratory symptoms. Diagnosed asthma was reported in 31% (of whom 42% were diagnosed aged 2 years or under) and asthma medications used in the previous year by 21% of children. Factors significantly associated with a reported asthma diagnosis included: male gender (OR: 1.51), birth in Australia (OR: 1.64), living in an English speaking household (OR: 1.47), Aboriginality (OR: 2.32), possession of a health care card (OR: 1.28), previous pneumonia (OR: 2.4) or pertussis (OR: 2.0), and a recent episode of croup (OR: 1.9). Exposure to tobacco smoke and immunisation status were not significant. CONCLUSIONS We confirm a high prevalence of asthma and medication use for asthma. The high proportion of children diagnosed asthmatic at 2 years or under (when asthma cannot be diagnosed reliably) suggests overdiagnosis of asthma may contribute to the apparent high prevalence.