Despite considerable research, the aetiology of asthma and allergic disease remains poorly understood. The International Study of Asthma and Allergies In Childhood (ISAAC), was founded to maximize the value of epidemiological research into asthma and allergic disease by establishing a standardized methodology and facilitating international collaboration. It has achieved its specific aims which are to describe the prevalence and severity of asthma, rhinitis and eczema in children living in different centres and to make comparisons within and between countries; to obtain baseline measures for assessment of future trends in the prevalence and severity of these diseases; and to provide a framework for further aetiological research into genetic, lifestyle, environmental and medical care factors affecting these diseases. The ISAAC design comprises three phases. Phase One used simple core written questionnaires for two age groups, and was completed in 156 collaborating centres in 56 countries and a total of 721 601 children participated. In the 13-14 years age group 155 centres from 56 countries participated, of which 99 centres completed a video questionnaire. For the 6-7 years age group there were 91 collaborating centres in 38 countries. ISAAC Phase One has demonstrated a large variation in the prevalence of asthma symptoms in children throughout the world including hitherto unstudied populations. It is likely that environmental factors were responsible for major differences between countries. The results provide a framework for studies between populations in contrasting environments which are likely to yield new clues about the aetiology of asthma. ISAAC Phase Two will investigate possible aetiological factors, particularly those suggested by the findings of Phase One. ISAAC Phase Three will be a repetition of Phase One in the year 2000 to assess trends in prevalence.