Asthma mortality among young people varies widely between different countries. Geographical differences in asthma prevalence are also believed to exist, but evidence is sparse owing to the lack of multicentre surveys using common protocols. A survey was conducted of 12-year-old children living in defined areas of New Zealand, Wales, South Africa and Sweden, in order to see whether asthma prevalence and mortality rates in children show parallel differences. Questionnaires enquiring about a history of asthma and respiratory symptoms were issued to the parent. The children performed a simple exercise challenge test. Information was obtained for 4353 children. A history of asthma at any time was reported for 16.8% of children in New Zealand, 12.0% in Wales, 11.5% in South Africa and 4.0% of Sweden, and a similar pattern was shown by several other indices of asthma (vanous relevant symptoms, inhaler use, response to exercise challenge, and asthma mortality at ages 5–19 years). In Sweden wheezing was negatively associated with pet ownership; elsewhere there was a positive (though non-significant) association. Cat ownership was highest in New Zealand and lowest in Sweden. The prevalence of asthma in children shows geographical variation which is parallel to that of asthma mortality, being high in New Zealand and low in Sweden. Differential exposure to animal allergens is a possible factor in this variation.