In an attempt to prevent or reduce the manifestations of atopic disease, a group of infants considered to be genetically at high risk of atopy was entered in a prenatally randomized, controlled study. A prophylactic group (n = 58) was either breast-fed with their mothers excluding foods regarded as highly antigenic from their diets, or given an extensively hydrolysed formula. In addition, strenuous efforts were made to reduce exposure to the house-dust mite by application of acaricide to the bedroom and living room carpets and upholstered furniture. A control group (n = 62) was fed conventionally by breast or on formula, and no specific environmental measures were taken. The results (previously reported) after 1 year showed significantly less total allergy, asthma, and eczema in the prophylactic group. Similar results were obtained at 2 years although the reduction in asthma no longer achieved statistical significance. However, there was significantly less sensitization, as shown by a battery of skin prick tests (SPTs), to both dietary allergens and aeroallergens in the prophylactic group. All the children have now been reviewed at the age of 4 years, and SPTs to a wide range of dietary allergens and aeroallergens have been performed. The control group continues to show more total allergy (odds ratio [OR] 2.73, 95% confidence interval [CI] 1.21-6.13, P < 0.02), definite allergy (allergic symptoms plus positive SPT) (OR 5.6, CI 1.8-17.9, P < 0.005), and eczema (OR 3.4, CI 1.2-10.1, P < 0.05). More control children have positive SPTs (OR 3.7, CI 1.3-10.0, P < 0.02). A dual approach to the prevention of allergic disease, avoiding as far as possible sensitization to food and aeroallergens, significantly reduces the risk of atopic disease. This should be reserved for infants considered at very high risk of atopy, and close medical and dietetic supervision must be available.