The prevalence of allergic diseases in childhood is regularly increasing, in particular asthma, which morbidity is important. Despite the persistence of a significative bronchial inflammation the severity of the disease may present with discrete symptoms. The recognition of asthma and its treatment is then insufficient and consequently potentially fatal asthma attacks may occur and there is a chronic remodeling of the airways. Allergenic sensitization occurs mainly during the first years of life and is related to the environment to which the child is exposed. The effect of early prevention was extensively studied in the past. A regimen with hydrolyzed cow's milk proteins during the first months of life decreases the incidence of milk allergy and atopic dermatitis, but does not seem to decrease the long term prevalence of allergies. Infectious agents are suspected to influence the type of cytokines produced by helper T cells and thus to promote or to prevent allergen sensitization. It has been shown recently that the influence of the environment during the first years of life is critical for the development of the IgE responses to allergens and as consequence for allergic manifestations. A better understanding of these mechanisms should allow to determine the reason(s) for the increase of allergic prevalence and improve the efficacy of preventive measures.