During the 1990's the international variation in the prevalence of asthma and allergy was studied in young adults (European Community Respiratory Health Survey, ECRHS) and in children (International Study of Asthma and Allergy in Children, ISAAC). The prevalence of asthma and allergy was highest in the United Kingdom, North America, Australia and New Zealand, lower in the Baltic countries and very low in some of the poorest republics of the former USSR. Children that lived in the countryside during the first year of life had a lower prevalence of atopic disease as teenager while in adults farmers had the highest prevalence of occupational asthma. In both investigations a negative association was found between having pets as a child and atopic sensitisation. This association could, however, be related to selection mechanisms. In a three year follow-up of the Swedish part of the ECRHS the incidence of asthma was higher in smokers than non-smokers. Passive smoking was associated with respiratory symptoms in both children and adults. Follow-ups of both studies are being planned or performed. Further analyses of the geographic differences may explain the reason for the rapid increase of asthma and allergy in the Western world.