The occurrence of allergic diseases in children was studied on the basis of a questionnaire sent to the parents of 20,000 school children, 7, 10 and 14 years of age, in 3 parts of Sweden with different climatic conditions. The prevalence of asthma was 2.4%, allergic rhinoconjunctivitis 7.4%, eczema 7.8% and total allergic diseases 16.9%. The prevalence of all diseases was significantly higher in the northern part of the country than in the southern parts. This geographic variation was not related to heredity, infant feeding pattern or known exposure variables other than the cold and dry climate. Parental history of allergic diseases increased the incidence in the offspring 2-9 times, with a pattern of symptom specificity and a cumulative effect of double parental history. Breast-feeding postponed the onset of allergic disease only in children with double parental history.