The International Study of Asthma and Allergies in Childhood (ISAAC) was designed to allow comparisons between populations in different countries. ISAAC Phase One, reported here, used standardized simple surveys which were conducted among representative samples of school children from centres in most regions of the world. Two age groups (13-14 and 6-7 yrs) with approximately 3,000 children in each group were studied in each centre. The 13-14 yr olds (n=463,801) were studied in 155 centres (56 countries) and the 6-7 yr olds (n=257,800) were studied in 91 centres (38 countries). There were marked variations in the prevalence of asthma symptoms with up to 15-fold differences between countries. The prevalence of wheeze in the last 12 months ranged from 2.1-32.2% in the older age group and 4.1-32.1% in the younger age group and was particularly high in English speaking countries and Latin America. A video questionnaire completed in the older age group in 99 centres (42 countries) showed a similar pattern. The major differences between populations found in the International Study of Asthma and Allergies in Childhood Phase One are likely to be due to environmental factors. The results provide a framework for studies between populations in contrasting environments which are likely to yield new clues about the aetiology of asthma.