Atopy development cannot yet be predicted with certainty, and primary prophylaxis has to be directed at every 'at-risk' infant. However, substantial progress has been made in identifying environmental factors associated with allergic disorders. The process of sensitisation to common allergens may begin in utero. The influence of adjuvants, especially passive smoking, is considerable in individuals with established atopic disease. This article examines the role of intervention during pregnancy and the first year of life in the primary prevention of atopy and suggests a number of measures based on the current evidence.