The Swiss SAPALDIA study is a large multicenter cross-sectional study initiated in 1991 to evaluate the relationship between environment and respiratory symptoms and diseases in adults, and included subjects from eight areas in Switzerland with distinctive environmental characteristics. We present here prevalence data for atopy, pollinosis and atopic asthma obtained from a random sample of 8,357 adults (18-60 years) assessed by standardized computer-based interview as well as by allergy skin prick tests (SPTs) (performed with Phazet) to grass, birch and Parietaria pollen, house dust mite, cat and dog epithelia and the moulds Alternaria and Cladosporium and by an in vitro allergy screen test (Phadiatop CAP FEIA system). On the basis of a positive Phadiatop (total 28.9%; males 32.9%, females 25.0%; p < 0.001) and/or a positive SPT (total 23%; males 25.0%, females 20.8%; p < 0.001), 32.3% of the study population were considered atopic (males 35.7%, females 28.8%; p < 0.001). Concerning the prevalence of skin sensitization (SPT wheal > or = 3 mm), the highest rate was observed for grass (12.7%), followed by house dust mite (8.9%), silver birch (7.9%), cat (3.8%) and dog (2.8%), whereas moulds and Parietaria elicited less than 1% positive SPTs. The prevalence of atopic rhinitis (rhinitis symptoms associated with atopy) was 13.5% (males 14.3%, females 12.6%; p < 0.05) and the prevalence of current hay fever varied between 9.1% (questionnaire answer and a positive SPT to at least one pollen), 11.2% (questionnaire answer and presence of atopy) to 14.2% (questionnaire answer only) with no significant difference by sex.(ABSTRACT TRUNCATED AT 250 WORDS)