Dietetic treatment has important supportive function in dogs and cats with neoplastic diseases. First goal is to supply enough energy and nutrients to avoid or retard weight loss and thereby maintain fat and protein reserves. Diets should be formulated with a high energy density. Higher fat concentrations and lower carbohydrate content is recommended compared to standard products. Supportive effects can be expected by increased concentrations of n-3 fatty acids (fish oil), arginine, trace elements (zinc, selenium) and anti-oxidative vitamins (vitamin E), that are required for the function of the immune system.