BACKGROUND Preoperative mechanical bowel preparation is aimed to reduce the risk of infectious complications, and its utility is a dogma in left-sided large bowel anastomosis. The aim of this study was to specifically assess whether colocolonic and colorectal anastomoses may be safely performed without preoperative mechanical bowel preparation. METHODS Patients undergoing elective colon and rectal surgery with primary colocolonic or colorectal anastomosis were prospectively randomized into two groups. The "prep" group had mechanical bowel preparation prior to surgery, while the "non-prep" group had surgery without pre-operative mechanical bowel preparation. RESULTS Two hundred forty-nine patients were included in the study, 120 in the prep group and 129 in the nonprep group. Demographic characteristics, indications for surgery, and type of surgical procedure did not significantly differ between the two groups. There was no difference in the rate of surgical infectious complications between the two groups. Overall infectious complication rate was 12.5% in the prep group and 13.2% in the non-prep group. Wound infection, anastomotic leak, and intra-abdominal abscess occurred in 6.6%, 4.2%, and 1.6% of patients in the prep group and in 10.0%, 2.3%, and 0.7% of patients in the nonprep group, respectively (p=NS). CONCLUSIONS These results suggest that elective left-sided anastomosis may be safely performed without mechanical preparation. Multicenter studies to test the reproducibility of these results are required, to support a change in this time-honored practice.