Abstract Despite the common recommendations of the criteria for the diagnosis of asthma there is still a wide variation within different regions in diagnoses, use of medications and hospitalisation rates especially among young children. This thesis elucidates the role of spesified environmental risk factors associated with the development of atopic diseases in childhood. In two prospective follow-up surveys we found that allergies and asthma associate with the consumption of margarines, butter and fish and that the common infection of childhood, RS-virus infection, does not increase asthma morbidity in adolescence. In a randomised set-up we were able to verify that the common childhood infections do not protect from allergies and asthma. In a retrospective survey we found that hospitalisation rates can reflect medication practices in different regions. Our results indicate that consumption of fat in the diet can be one triggering factor for allergies but common childhood infections are merely markers of susceptibility to allergies and asthma rather than the cause of it.