Food allergy is an abnormal response of the body’s immune system to food components. The proportion of patients with food allergies is considered to be increasing to about 10% in the general population in the developed countries. In countries of the European Union adverse reactions caused by soybean formulas are seen in 14 to 35% of cow’s milk–allergic infants. In food-allergic subjects with atopic dermatitis, milk, egg, soybean, peanut, and wheat account for almost 90% of allergic reactions. The allergenicity of soybean resides in the protein fractions, not in soybean oil itself, whereas oxidized soybean oil has been shown to enhance the immunoglobulin E-binding ability against soybean or other food proteins. In the United States, 28% of food-allergic patients with atopic dermatitis were reported to be soybean positive by a double-blind, placebo-controlled food challenge test. The allergen was hardly detected in fermented products, such as soybean paste (miso) and fermented soybean (natto).