Patients undergoing major gastrointestinal surgery are potentially at high risk of malnutrition, and oral nutritional supplement solutions have been commercialized as an easily edible balanced nutritional source. A multicenter prospective randomized study to examine the efficacy of postoperative oral nutritional supplementation is ongoing in Korea. A total of 174 patients in eight institutes who are supposed to undergo major gastrointestinal organ resection or significant bowel reconstruction will be prescreened before the operation, and randomized to either the oral nutrition supplementation group or the control group when they are discharged after the operation. After discharge, patients in the oral nutrition supplementation group take 200 ml of Encover? solution twice per day for eight weeks, thereafter the weight loss rate of each group compared with preoperative body weight will be compared with that of the control group as a primary endpoint. The secondary endpoints include weight loss rate compared with that at the time of discharge, changes of body mass index, biochemical serum parameters, patient-generated subjective global assessment, and anthropometric measurements (ClincalTrials.gov ID: NCT01838109). (SMN 2013;4:31-36)