Background: Perioperative oral supplementation has been shown to reduce post-operative complications. However, the use of preoperative standard oral supplements in a cohort of colorectal cancer patients has not been evaluated. The present study examined whether preoperative supplements are beneficial in this group. Methods: In a randomised controlled trial, patients were assigned to receive 400mL of oral supplement and dietary advice or dietary advice alone. Primary outcome was the number of post-operative complications. One hundred and twenty-five patients were recruited (59 randomised to the intervention group and 66 to the control group) and nine were excluded. Results: In the intervention group, 24 (44%) patients had a complication compared to 26 (42%) in the control group (P=0.780). In the intervention and control groups, there were eight (15%) and 16 (25%) surgical site infections, respectively (P=0.140) and seven (13%) and 11 (17%) chest infections, respectively (P=0.470). Subgroup analysis for hypothesis generation included 83 (71%) weight-losing patients, where there was a significant reduction in surgical site infections using the Buzby definition (P=0.034), although this was not the case for the Centre for Disease Control definition (P=0.052). Conclusions: There was no evidence that preoperative supplements were beneficial in reducing the number of complications, although there may be some benefit for surgical site infections in selected weight-losing preoperative patients. © 2011 The Authors. Journal of Human Nutrition and Dietetics © 2011 The British Dietetic Association Ltd.