Allergy was probably rare at the beginning of this century, but its prevalence in developed countries has now risen to true epidemic proportions. There has been about a doubling of children with allergic rhinitis in the past twenty years. This increase can only be due to a factor in the environment, and the identification of these factors should be a research priority in order to reverse the trend and to make atopy a preventable disease. Air pollution is unlikely to be a major contributor, while more attention is turning towards the influence of living conditions, hygiene, family size, vaccinations, infections and use of antibiotics. An explanation in immunologic terms is found in the need for external input in the maturation of our immune system, especially the need for local triggering in the respiratory and alimentary tract. This seems to be required to reverse the dominance of allergy-promoting Th2 cytokine patterns at birth into a balanced equilibrium between Th1 and Th2 cell responses upon antigenic triggering. On the basis of these epidemiologic and immunologic data, guidelines are emerging for allergy prevention.