Nutritional support of cancer patients should ideally contribute to improve the immune defence, maintain the protein body pool and sensitize the tumour to oncologic treatments. Such a goal is not easy to achieve, because any nutritional support efficient at stimulating immune response and protein syntheses might also simultaneously stimulate the tumour growth. Contradictory observations have been reported for several nutrients contained in the nutritive solutions available on the market. This is the case for glutamine, arginine, omega-3 fatty acids and nucleotides. Their clinical use during oncologic therapies deserves further testing and analysis.