Atopy is the state of allergy to common environmental antigens. Genetic and environmental factors promote the disorder. The impressive rise in prevalence, mainly centred on socio-economically developed communites around the world, emphasizes the potent action of environmental factors in moulding this immune disorder which is characterized by inadequately restrained Th-2 immune mechanisms and IgE production. Reversing the epidemiological trend depends on our identifying the major environmental inputs and acting against these. As yet, the nature of these environmental factors remains to be clarified. Candidate factors include changes in diet, chemical air pollution and microbial exposures in developed countries. This article limits its scope to changing microbial exposures as a potential mechanism. (a) It records epidemiological data that have associated atopic status with less natural exposure to pathogens, parasites and commensal micro-organisms, but with more exposure to certain antibiotics and public health immunizations in early life. (b) It records studies in mice that support the concept that certain microbial exposures can inhibit experimental allergy. (c) It considers potential immune mechanisms for such an action, including the possibility that certain natural infections promote immune regulatory processes that can restrain atopy. It is concluded that the hypothesis that changing patterns of microbial exposure may have promoted the rise in atopy is viable, and that exciting possibilities for reversing the rise of atopy may be derived from further studies.