The theory of the adenoma-carcinoma sequence suggests an adenoma progresses a full carcinoma. Aim: To determine if the rate of synchronous cancer increases in advanced cancers such as obstructive distal colon cancer. Method: The medical records (1999-2002) of the 386 consecutive patients with distal colon cancer who underwent an intraoperative (obstruction; n=80) or preoperative(non-obstruction; n=306) colonoscopy at our hospital were retrospectively reviewed. An intraoperative colonoscopy was performed after an on-table lavage for cases in which colon cancer prevented the proximal bowel from being examined preoperatively. A new device, which enables an easy on-table lavage and a subsequent colonoscopy before resecting the tumor, was used for this study. RESULTS: The obstruction and non-obstruction groups had similar demographics. The TNM stage of the obstruction and non-obstruction group was I 0(0%), II 27(34.2%), III 33(41.3%), IV 20(25.3%) and I 56(18.3%), II 127(41.5%), III 102(33.3%), IV 21(6.9%). No significant difference was observed between the groups (obstruction vs non-obstruction) in rates with synchronous polyps(57.5 vs 60.8%; p=0.87) or synchronous cancer(12.5 vs 7.8%; P=0.24). CONCLUSIONS: These results do not suggest that an increased synchronous cancer appears in an obstructing distal colon cancer. However, because of the frequently associated neoplasm, all patients with a colorectal cancer obstruction are advised to undergo a full colonoscopy intraoperatively.