Since January 2002, the European Commission is funding a large project, 'Protection against Allergy--Study in Rural Environments' (PASTURE; contract no. QLK4-2001-00250), under the Fifth Framework Program in the field of epidemiology of allergic diseases. The aim of this paper was to describe the background and design as well as the aims of the project. Asthma and allergic disorders are a major public health problem in many Western countries. The aetiology of asthma and allergic disease remains poorly understood despite considerable research. Epidemiology has the potential to add greatly to the understanding by elucidating the risk factors for asthma and allergic disease and thereby suggesting productive avenues for research into causation and prevention. Several risk factors for the development of asthma and atopic disease in children such as passive smoke exposure during pregnancy and infancy, low birth weight or high body mass index later in life have been described. Furthermore, there is consistent evidence that the prevalence of atopy increases with higher socio-economic status. Levels of air pollution such as ozone, NO2, SO2 and particles are likely to provoke acute exacerbations of pre-existent respiratory disease. Their role in the inception of asthma and allergies remains to be clarified. Allergen exposure has been linked to the development of atopic sensitization to that particular allergen in children as well as in adults with occupational exposures. Exposure to house dust mite or cat allergen is, however, unlikely to contribute to the development of childhood asthma. In turn, pet keeping in the first year of life, particularly, dog keeping, has been inversely related to the development of wheeze and atopic illnesses. Several prospective birth cohort studies found a decreased prevalence of atopic disease in children having daily contact to pets, in particular to cats and dogs, during early infancy. The protective effect might be attributable to allergen or other exposures associated with pet ownership, but may also in part be because of the removal of pets in families with sensitized or symptomatic children or in families with a positive history for atopy at the time the child was born.