Some nutrients, such as glutamine, arginine, omega-3 fatty acids and nucleotides have been shown to modulate the immune and intestinal systems. They affect the synthesis of proteins and the expression of enzymes, as well as the activation and the proliferation of leukocytes and enterocytes. Since these immunonutrients modulate the response of tissues with rapid cell turnover, such as spleen, thymus, bone marrow and intestinal mucosa, they may also affect, such as suggested by some preliminary results, cancer cell proliferation and tumour growth. Therefore, further investigations are needed to understand the underlying mechanisms and to determine possible advantages and limitations of their administration to cancer patients.