A total of 149 patients admitted for elective colorectal surgery were randomly allocated to receive preoperative mechanical bowel preparation (group 1) or no mechanical bowel preparation (group 2). All patients received antimicrobial prophylaxis with cephalothin and metronidazole. The overall incidence of wound infection was 17.4 per cent (24 per cent for group 1, 12 per cent for group 2) and that of dehiscence 7.4 per cent (10 per cent for group 1, 5 per cent for group 2). The incidence of wound infection was significantly higher in group 1 (P < 0.05) but that of anastomotic dehiscence did not differ significantly between groups. Mechanical bowel preparation is unnecessary and may be harmful in terms of preventing wound infection and anastomotic dehiscence in patients undergoing elective colorectal surgery.