Malnutrition is a well-known risk factor significantly influencing the occurrence of postoperative infectious complications. There is consensus that nutritional support is an essential component of the multidisciplinary treatment of surgical and critically ill patients. Nutritional support of surgical patients can be carried out with different modalities, depending on the underlying disease and on the patient's general condition. Several studies have shown that the early administration of enteral nutrition promotes the restoration of gastrointestinal mucosa integrity; with total parenteral nutrition such beneficial effect is not observed. The timing of feeding also influences the clinical outcome. During the last few years, standard enteral preparations have been modified by the addition of immunonutrients, such as arginine, glutamine, omega-3 fatty acids, nucleotides and others. These substrates have been shown to up-regulate host immune responses, to control inflammatory responses and to improve nitrogen balance and protein synthesis after injury. A recent study reported that in patients with cancer of the gastro-intestinal tract the nutritional supplementation given only preoperatively was as effective as the combined pre- and postoperative (perioperative) approach, and it could reduce gastro-intestinal side effects. This is probably due to the effect of the immune-enhancing diet on the immune and inflammatory responses.