Phase One of the International Study of Asthma and Allergies in Childhood (ISAAC) has reported the prevalence of asthma, rhinitis and eczema symptoms in children. In 99 centres from 40 countries, a total of just under 317,000 13-14-yr-old children also completed a video questionnaire, showing the symptoms and signs of asthma. This first video sequence has been compared to the ISAAC written question asking about current wheezing to explore variations in agreement and the contribution of each questionnaire to wheezing prevalence between centres, by region and language groups. In general, responses to the video questionnaire gave a lower prevalence than the siritten questionnaire and responses were closely correlated. The overall proportion of agreement was high, mean 0.89. but unbalanced, with good negative agreement but poor positive agreement. Chance corrected agreement using Cohen's kappa coefficient, was generally low, with onl 20 centres with kappa >0.4. The contribution of each questionnaire to wheezing prevalence also varied between centres and suggests that written questions about wheezing are variably understood and interpreted by 13-14 yr olds. International comparisons of wheezing and its audiovisual presentation suggest that adolescents interpret a written question about wheezing differently from its audiovisual presentation and that this interpretation shows variation between centres. This relationship and the interpretation of both written and audiovisual presentation of symptoms requires further study in order to better predict asthma.