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Background: Variations in the prevalence of atopy could provide important clues to the etiology of atopy and asthma. Although estimates of prevalence are available from different studies, a lack of standardization makes comparisons difficult. Objective: This study was conducted to estimate the variation of geometric mean levels of serum IgE and the prevalence of specific IgE to common allergens between populations as part of the European Community Respiratory Health Survey (ECRHS), a multicenter survey of asthma and risk factors for asthma. Methods: Random samples of subjects living in 37 centers in 16 countries who had answered a questionnaire about their respiratory symptoms were invited for further assessment including total serum IgE and the presence of specific IgE against house dust mite (Dermatophagoides pteronyssinus) , timothy grass, cat, Cladosporium herbarum, and a local allergen. Sera were tested from 13,883 persons. Results: The estimated prevalence of atopy, defined as the presence of at least one positive specific IgE, ranged from 16% in Albacete (Spain) to 45% in Christchurch (New Zealand). The geometric mean total serum IgE varied from 13 kU/L in Reykjavik (Iceland) to 62 kU/L in Bordeaux (France). There was no relation between the geometric mean total serum IgE in a center and the prevalence of atopy. Conclusions: There are substantial variations in the prevalence of atopy and the level of serum IgE. These variations are independent of each other and likely to be largely environmental in origin. (J Allergy Clin Immunol 1997;99:314-22.)