Pregnancy is associated with a strong skewing toward T(H)2 cytokine pattern, which enables the survival of the fetus, including fetal allergen-specific immune responses. The postnatal maturation of the immune system which is characterized by the development of a balanced T(H)1/T(H)2 immunity is genetically determined and modified by the environment. The process seems to proceed at a slower rate in atopic than in nonatopic infants. There is a close immunologic interaction between the mother and her offspring through the breast milk. Individual variations in the composition of human milk may explain the controversy with regard to the possible allergy-preventive effects of breast-feeding. Recurrent respiratory infections have been suggested to enhance immune deviation. The microbial flora are a more likely source, however, because they are a major driving force in the maturation of the immune system. Changes in its composition, as a consequence of an altered lifestyle and diet, may play a role in the higher prevalence of allergy. So far, primary prevention of allergy has failed. Future studies should therefore focus on factors enhancing immune deviation (ie, "success" factors) rather than on "risk" factors. The intestinal microflora is one of these factors that deserves closer analysis.