The Authors have studied the role of various preventing diet for a primary prophylaxis of allergy in 125 newborns at risk of atopy: 30 exclusively breast-fed, 50 hypoallergenic milk fed, 30 soy milk fed, and 15 with conventional milk formula. IgE values were determined at 5 days, 6 months, and 12 months of age, IgE values at 5 days were compared to newborns not at atopic risk. The clinical follow-up lasted 4 years. Total IgE values at 5 days were significantly higher in new-born at atopic risk. Only breast-feeding subjects had IgE normal values at six months. Allergic symptoms were observed in 14% of infants with a guided diet and in 53% of infants with a conventional diet. Breast fed subjects had atopic disorders in only 8% of cases, subjects with hypoallergenic formula in 12% while soja milk fed in 25%. The Authors stress the role of breast feeding in preventing allergic disorders in subjects at atopic risk or, when human milk misses, of a hypoallergenic formula, more than soy milk and conventional formula and confirm the possibility of diet and ambiental prophylaxis of allergy.