After reviewing our current knowledge of the metabolic response to major abdominal surgery, we present two new studies in which we have followed the changes in body composition, physiologic function, and psychological function for up to 1 year after major surgery. In the first study, 46 patients had changes in body composition, physiologic function, and psychological function measured perioperatively and again at intervals up to 1 year. There was an average loss of weight of 3000 grams (maximum at 2 weeks), and the tissue composition of this weight loss was composed of 1400 grams fat, 600 grams protein, and 1000 grams water. Normal body composition had returned to normal in most patients by 6 months. In the second study of 84 patients, deficits of skeletal muscle function and fatigue were found to be greatest at postoperative day 7. Most patients were back to preoperative levels of muscle function and fatigue 30 days after surgery, and nearly all had normal values 90 days after surgery. Based on these studies we outline our management program for the metabolic care of patients undergoing major abdominal surgery.