To examine the effects of maternal diet during lactation and the use of formula feeds on the development of atopic eczema in infants at high-risk, a group of 97 mothers who chose to breastfeed and 124 mothers who did not were examined at a child health center in Canada. Mothers who chose to breastfeed exclusively were randomly allocated to either a restricted diet (avoiding milk and other dairy products, eggs, fish, peanuts, and soybeans) or a diet without restrictions. Mothers who did not plan to breastfeed were randomly allocated to the use of 1 of 3 formula feeds. Their choices were either casein hydrolysate formula, soy milk formula, or cows' milk formula. Infants were followed for over 18 months and examined for eczema. Eczema was less common and milder in babies who were breastfed and whose mothers were on a restricted diet (11 of 49 [22%] vs 21 of 48 [48%]). In infants fed the casein hydrolysate formula, soy milk, or cows' milk, 9 of 43 (21%), 26 of 41 (63%), and 28 of 40 (70%), respectively, developed atopic eczema. In families with a history of atopic eczema, mothers who breastfed should avoid common allergenic foods during lactation. If they choose not to breastfeed, a hydrolysate formula should be used.