Studies have presented conflicting associations between cat allergen exposure and sensitisation and atopic disease. We therefore investigated the association between the observed domestic cat allergen level and cat sensitisation in young children in four study populations from three European countries. We recruited children from a nested case-control study, which is composed of four ongoing birth cohorts conducted in three European countries. Children at 2-4 years of age in the four cohorts who were sensitised to cat allergens (n=106) were compared with 554 non-sensitised children (controls). House dust samples were collected when the children were 5 to 7 years old, and cat allergen levels were measured in ng/g dust and ng/m(2) surface area. In the German study population we found a positive association between domestic cat allergen in house dust and cat sensitisation (OR (CI)=3.01 (1.16, 7.99)) while in the Swedish study population, we found a negative association (OR (CI)=0.41 (0.16, 0.98)). No association was found in the Dutch study population (OR (CI)=0.83 (0.22, 2.93)). Looking into the family history of cat keeping, we found the lowest prevalence of cat sensitisation in children who were cat owners at the age of blood sampling (11%) and the highest prevalence was found in those who have had a cat but not anymore, at the age of blood sampling (41%). The mixed results may be explained by differences in age and avoidance patterns.