The effect of maternal and infant avoidance of allergenic foods on food allergy was examined in a prenatally randomized, controlled trial of infants of atopic parents. The diet of the prophylactic-treated group (N = 103) included (1) maternal avoidance of cow's milk, egg, and peanut during the third trimester of pregnancy and lactation and (2) infant use of casein hydrolysate (Nutramigen) for supplementation or weaning, and avoidance of solid foods for 6 months; cow's milk, corn, soy, citrus, and wheat, for 12 months; and egg, peanut, and fish, for 24 months. In the control group (N = 185), mothers had unrestricted diets, and infants followed American Academy of Pediatrics feeding guidelines. The cumulative prevalence of atopy was lower at 12 months in the prophylactic-treated (16.2%) compared to the control (27.1%) group (p = 0.039), resulting from reduced food-associated atopic dermatitis, urticaria and/or gastrointestinal disease by 12 months (5.1% versus 16.4%; p = 0.007), and any positive food skin test by 24 months (16.5% versus 29.4%; p = 0.019), caused primarily by fewer positive milk skin tests (1% versus 12.4%; p = 0.001). The prevalences of allergic rhinitis, asthma, and inhalant skin tests were unaffected. Serum IgE levels in the prophylactic-treated group were marginally lower only at 4 months. Thus, reduced exposure of infants to allergenic foods appeared to reduce food sensitization and allergy primarily during the first year of life.