Environmental factors which have changed in the last decade or so appear to be largely responsible for the increase in the prevalence of asthma in affluent countries. It should, therefore, be possible to design interventions to reverse these recent trends and reduce the incidence of asthma. Primary preventive strategies have the potential not only to reduce acquisition of sensitization to common allergens and the risk that symptoms will develop subsequently, but also to reduce morbidity in those who already have persistent disease. There is accumulating epidemiological evidence that a dietary exess of sodium and omega-6 fatty acids, a dietary deficiency of antioxidant vitamins and omega-3 fatty acids, reduced rates of breastfeeding and exposure to allergens and environmental tobacco smoke are all involved in the aetiology of asthma. The modification of these factors has the potential to reduce the incidence and thus the prevalence of this disease. Environmental intervention should be particularly effective in children who have inherited or acquired characteristics which put them at high risk of developing asthma. With the evidence now available, it seems reasonable to assume that interventions which are based on our current knowledge of risk factors could achieve a 50% reduction in the prevalence of asthma in the next generation of children.