Atopic diseases constitute a common health problem. For infants at hereditary risk, prophylaxis of atopy has been sought in elimination diets and other preventive measures. We followed up healthy infants during their first year, and then at ages 1, 3, 5, 10, and 17 years to determine the effect on atopic disease of breastfeeding. Of the initial 236 infants, 150 completed the follow-up, which included history taking, physical examination, and laboratory tests for allergy. The subjects were divided into three groups: prolonged (> 6 months), intermediate (1-6 months), and short or no (< 1 month) breastfeeding. The prevalence of manifest atopy throughout follow-up was highest in the group who had little or no breastfeeding (p < 0.05, analysis of variance and covariance with repeated measures [ANOVA]). Prevalence of eczema at ages 1 and 3 years was lowest (p = 0.03, ANOVA) in the prolonged breastfeeding group, prevalence of food allergy was highest in the little or no groups (p = 0.02, ANOVA) at 1-3 years, and respiratory allergy was also most prevalent in the latter group (p = 0.01, ANOVA) having risen to 65% at 17 years of age. Prevalences in the prolonged, intermediate, and little or no groups at age 17 were 42 (95% CI 31-52)%, 36 (28-44)%, and 65 (56-74)% (p = 0.02, trend test) for atopy, respectively, and 8 (6-10)%, 23 (21-25)%, and 54 (52-56)% (p = 0.0001, trend test) for substantial atopy. We conclude that breastfeeding is prophylactic against atopic disease--including atopic eczema, food allergy, and respiratory allergy--throughout childhood and adolescence.