Adverse reactions to foods involving abnormal immune reactions to food antigens occur in 2-7% of the North American population; the numbers are perhaps higher in children. Both IgE-mediated and non-IgE-mediated allergic responses occur. IgE-mediated allergic responses to foods are the most dramatic and perhaps the most easily diagnosed type of food allergy. Non-IgE-mediated food hypersensitivity is more chronic, less acute, less obvious in its clinical presentation, and often more difficult to diagnose. It usually presents in infants between one week and three months of age with vomiting and diarrhea, although irritability, poor feeding, and failure to thrive are not uncommon. A thorough history and physical examination are often key in establishing a diagnosis of food protein hypersensitivity. In non-IgE-mediated disease, skin tests and immunological studies are not helpful. Eliminating the food allergen is the only means of dealing with a food allergy in most patients. About 85% of infants who have formula protein intolerance will outgrow their symptoms somewhere between 1 month and 3 years of age, older children and adults are somewhat less likely to lose their sensitivity, although approximately one-third will after 1-2 years of dietary restriction.