Food intolerance can be caused by immunologic, pharmacologic, toxic, infectious, idiosyncratic, metabolic, and neuropsychologic processes. Food allergy denotes an adverse reaction to food or food additives in which an underlying immunologic mechanism can be shown. Its incidence in young children is approximately 1.3% and among adults is 0.3%. Parental history of atopy is a significant causal factor. In addition, exposure to common allergenic foods in infancy increases risk. For these reasons, exclusive breast-feeding and maternal avoidance of peanut, egg, fish, and dairy products during lactation has been recommended and shown to reduce the occurrence of food allergy. Wheat, egg, and fish should not be introduced until the infant is aged > 12 mo and peanut until the age of 36 mo. These measures and other environmental precautions can be expected to reduce the cumulative prevalence of allergy.