717 Background: A randomized controlled trial to confirm the non-inferiority of laparoscopic surgery to open surgery for clinical stage II/III colon cancers in terms of overall survival was conducted. In this ancillary study, we explored the risk factors for postoperative complications of laparoscopic surgery and open surgery. Methods: Eligibility criteria included colon cancer; tumor located in the cecum, ascending, sigmoid, or rectosigmoid colon; T3 or T4 without involvement of other organs; N0-2; and M0. Postoperative complications which were observed from the end of the operation to discharge were graded according to the CTCAE 3.0. Multivariate analysis was performed using logistic regression model. Results: Between October 2004 and March 2009, a total of 1,057 patients from 30 Japanese centers were registered. By per-protocol set, 524 patients underwent open surgery (OPEN) and 533 patients underwent laparoscopic surgery (LAP). Proportion of any grade (G) complication was 18.3% (OPEN 22.3%, LAP 14.3%), G2–G3 was 12.9% (OPEN 13.9%, LAP 11.8%), G3 was 5.3% (OPEN 6.9%, LAP 3.8%) and G4 was none. Postoperative complications (G2-G3) included leakage (OPEN 2.1%, LAP 1.9%), ileus (OPEN:1.5%, LAP:0.9%), and wound complication (OPEN: 0.2%, LAP: none). Multivariate analysis revealed that risk factors for postoperative complications were operation times 240 min or more (p=0.0019, odds ratio [OR] 2.01 [95% CI: 1.30-3.13]) and open surgery (p=0.0001, OR 2.05 [95% CI: 1.41-2.98]). Conclusions: Operation times more than 240 min and open surgery were considered to be the risk factors for postoperative complications for clinical stage II/III colon cancers. Clinical trial information: C000000105.