Preoperative malnutrition is often associated with poor postoperative outcome, yet there is no consensus about whether perioperative nutritional support reduces postoperative complications to the level occurring in well-nourished patients undergoing similar procedures. This is partly because reports evaluating effect of perioperative nutritional support on postoperative outcome vary widely in number of patients studied, primary diagnosis, and duration and quality of perioperative nutritional support. These concerns warrant caution in interpreting reported results, even of randomized studies. However, analysis of published reports suggests that when total parenteral nutrition (TPN) is given to malnourished patients in adequate amounts for greater than or equal to 7-15 d preoperatively, significant improvements in both nutritional status and postoperative clinical outcome are likely to occur. Preoperative total enteral nutrition (TEN) is as effective as TPN in improving postoperative clinical outcome. Postoperative TPN, TEN, and ad libitum oral nutrition are equally effective in reducing postoperative complications. Potential candidates for surgery for whom prompt initiation of preoperative TPN or TEN may reduce operative morbidity and mortality irrespective of nutritional status can be identified on admission.