The Phase One International Study of Asthma and Allergies in Childhood (ISAAC) was developed for geographical and temporal comparisons of the prevalence and severity of asthma and allergic diseases in childhood within and between countries. In Thailand, a study was carried out in Bangkok and this is a further study undertaken in Chiang Mai. Using the ISAAC questionnaire, schoolchildren of two age groups were studied: 6-7 years old (n = 3,828) and 13-14 years old (n = 3,927). The data were entered and analyzed using the Epi-info computer program. The prevalence over the past 12 months, in 6-7 and 13-14 years olds, respectively, are as follows; wheezing, 5.5% and 12.6%; rhinitis, 18.5% and 38.3%; rash at flexural areas, 12.9% and 10.7%. The prevalence of these three conditions was found to be close to, but slightly lower than, that from Bangkok, except for eczema in older children.