We report an interim analysis of a prospective single-blinded randomized trial designed to investigate whether preoperative mechanical bowel preparation influences the rate of surgical-site infection and anastomotic failure after elective colorectal surgery with primary intraperitoneal anastomosis performed by a single surgeon. Patients scheduled to undergo an elective colorectal procedure with a primary intraperitoneal anastomosis were randomized to receive either oral polyethylene glycol lavage solution and enemas (group A) or no preparation (group B). Surgical-site infection and anastomotic failure were investigated. Of 97 patients included, 48 were assigned to group A and 49 to group B. Twelve (12.4%) developed wound infections, six in each group (12.5 vs. 12.2%; NS). Intra-abdominal sepsis was only seen in group A (n = 3, 6.3%). Anastomotic failure occurred in four patients in group A (8.3%) vs. two patients in group B (4.1%) (NS). The overall complication rate in group A was 27.1%, vs. 16.3% in group B. The number needed to harm was 9.3. Our interim analysis of a prospective single-blinded randomized trial suggests that a surgeon may have the same or even worse outcomes when mechanical bowel preparation is routinely used for colorectal surgery with primary intraperitoneal anastomosis.