The role of cat keeping on the promotion of allergies is discussed controversially. We investigated the associations between cat keeping, allergen exposure, allergic sensitization and atopic diseases in pre-school children. A total of 606 children (5- to 6-yr old) were studied in the course of the mandatory school entrance examination. Information on doctor diagnosed asthma and allergic rhinitis, pet keeping and confounders was obtained by questionnaire. The prevalence of atopic eczema was determined by dermatological examination, allergic sensitization was assessed by skin prick test, and the allergen exposure to cat allergen Fel d 1 was measured by a commercial wipe test. Cats were present in 16% of the households and results of the exposure categories (0-III) on cat allergen were 47.2%, 25.5%, 24.3% and 3.0% respectively. The prevalence of cat keeping increased significantly with exposure categories from 0.5% to 61.5% (ptrend < 0.001). Children (6.3%) were sensitized to cat allergen and sensitization rates increased also significantly with exposure categories from 3.0% to 15.4% (ptrend < 0.001). Children (9.3%) were diagnosed with atopic eczema and a positive history of asthma/rhinitis was given in 3.6% and 3.9% respectively. Sensitization to cat was associated with atopic eczema (23.3% vs. 7.4%; ORadj.= 3.8, CI: 1.4-10.8), asthma (12.5% vs. 3.7%; ORadj.= 4.9, CI: 1.1-21.2), allergic rhinitis (6.9% vs. 2.7%; ORadj.= 3.1, CI: 0.7-15.2) and any atopic disease (43.5% vs. 16.3%; ORadj.= 3.8, CI: 1.5-9.5). The data suggest a promoting effect of cat keeping for atopic diseases.