This study examined the clinical effects and cost of administration of oral supplements both before and after surgery. An RCT was conducted in three centres. Patients undergoing lower gastrointestinal tract surgery were randomised to one of four groups: group CC received no nutritional supplements, group SS took supplements both before and after surgery, group CS received postoperative supplements only, and group SC were given supplements only before surgery. Preoperative supplements were given from the time it was decided to operate to 1 day before surgery. Postoperative supplements were started when the patient was able to take free fluids and continued for 4 weeks after discharge from hospital. Data collected included weight change, complications, length of stay, nutritional intake, anthropometrics, quality of life and detailed costings covering all aspects of care. One hundred and seventy-nine patients were randomised, of whom 27 were withdrawn and 152 analysed (CC 44, SS 32, CS 35, SC 41). Dietary intake was similar in all four groups throughout the study. Mean energy intake from preoperative supplements was 536 and 542 kcal/day in the SS and SC groups, respectively; that 2 weeks after discharge from hospital was 274 and 361 kcal/day in the SS and CS groups respectively. There was significantly less postoperative weight loss in the SS group than in the CC and CS groups, and significantly fewer minor complications in the SS and CS groups, than in the CC group. There were no differences in the rate of major complications, anthropometrics and quality of life.