This paper looks at hydrolyzed protein diets and the effects they have on cats and dogs. Poor palatability, hypoosmotic diarrhoea, reduced nutritional value, and persistent allergenicity are the main potential problems of hydrolyzed protein diets used for the diagnosis of food allergy or the treatment of inflammatory bowel disease. Initial studies of these diets are promising, although protein source should be considered as antigenic sites may not be completely destroyed.