Our understanding of the interrelationship between nutrition, immunology, oncogenesis, and tumor growth is steadily increasing. Well known are the detrimental effects of malnutrition on host defenses. New work has shown that the addition of omega-3 polyunsaturated fatty acids or the amino acid arginine to the diet may significantly enhance immune function. These dietary manipulations have been shown to inhibit tumorigenesis and cancer spread in animal models, and form the basis for exciting new work in the prevention and treatment of cancer in humans.