BACKGROUND Recent studies have suggested that MBP does not lower the risk of postoperative septic complications after elective colorectal surgery. This randomized clinical trial assessed whether preoperative MBP is beneficial in elective colonic surgery. METHODS A total of 1505 patients, aged 18-85 years with American Society of Anesthesiologists grades I-III, were randomized to MBP or no MBP before open elective surgery for cancer, adenoma or diverticular disease of the colon. Primary endpoints were cardiovascular, general infectious and surgical-site complications within 30 days, and secondary endpoints were death and reoperations within 30 days. RESULTS A total of 1343 patients were evaluated, 686 randomized to MBP and 657 to no MBP. There were no significant differences in overall complications between the two groups: cardiovascular complications occurred in 5.1 and 4.6 per cent respectively, general infectious complications in 7.9 and 6.8 per cent, and surgical-site complications in 15.1 and 16.1 per cent. At least one complication was recorded in 24.5 per cent of patients who had MBP and 23.7 per cent who did not. CONCLUSIONS MBP does not lower the complication rate and can be omitted before elective colonic resection. BACKGROUND ISRCTN28535118 (http://www.controlled-trials.com).