Forty-eight children with a biparental history of atopic disease were followed from birth to 4 years of age. One group was fed soy and the other cow's milk from weaning to 9 months of age. Two-thirds of the children developed symptoms of atopic disease with no significant difference between the groups. No difference was found in the serum immunoglobulins (IgE antibodies, IgA, IgG and IgM) during the observation period. The soy fed children showed transiently lower levels of IgG antibodies to cow's milk but higher levels of IgG antibodies to soy protein. Six children showed cow's milk intolerance and a further five had symptoms possibly related to the use of cow's milk. Withholding cow's milk during the first 9 months did not reduce the incidence of symptoms of cow's milk intolerance from birth to 4 years of age. Thus, no benefit was found from replacing cows' milk with soy. A prolonged breast feeding seems most rational for infants at risk of developing atopic disease, even if the present study did not show evidence of a prophylactic effect of breast milk against the development of atopic disease.