Asthma is responsible for much morbidity and mortality, which might be decreased by increasing parental and patient knowledge about asthma. This report describes the development and validation of an asthma knowledge questionnaire for use in the assessment of asthma knowledge. Two groups of respondents were examined. One group was designated 'high knowledge' and the other 'low knowledge'. The questionnaire had a maximum possible score of 31. The mean score of 69 'high' knowledge parents was 25.3 (median 25, range 18-31). By contrast, the mean score of 69 'low' knowledge parents was 13.0 (median 13, range 0-21). The difference between the two groups was statistically significant (P less than 0.0001). The questionnaire results were reproducible (tau = 0.94, P less than 0.01). Demographic characteristics such as education and socio-economic status were not important confounding variables. Face and content validity of the questionnaire were based on reference to published experience. Concurrent validity was demonstrated by the ability of the questionnaire to distinguish low from high knowledge parents. The questionnaire should prove to be a valuable research tool for the assessment of asthma knowledge either as part of an educational intervention or in a clinical situation.