In selected malnourished patients, perioperative nutritional support can decrease the morbidity and mortality rates associated with major surgical procedures. Preoperative nutritional support should be delivered via the gastrointestinal tract whenever feasible, generally in the form of enteral diets, which can be given via a feeding tube or as a dietary supplement. Patients with a functional gut who cannot eat because of anorexia or upper gastrointestinal tract obstruction are candidates for preoperative tube feedings. Total parenteral nutrition should be the mainstay of nutritional support when the gastrointestinal tract cannot be used adequately. An improvement in nutritional indices (e.g., serum transferrin, lymphocyte count) may be associated with decreased perioperative morbidity, although the strength of this relation is not clear. In the absence of improvement in such indices, the duration of nutritional support required to decrease operative morbidity is unknown. Postoperatively, enteral tube feedings (delivered via a nasojejunal tube or feeding jejunostomy) should be provided to all preoperatively malnourished patients with a functional gastrointestinal tract who are unable to consume adequate calories orally. Postoperative TPN should be reserved for malnourished patients with a nonfunctional gut or for patients who develop a postoperative complication that precludes enteral feeding. Current nutritional formulas have often neglected the metabolic and nutritional requirements of the intestinal tract. In the future, the combined use of specific nutrients and growth factors may improve nutritional rehabilitation in catabolic patients.