The reported incidence of synchronous primary cancers and polyps associated with single cancers of the large bowel is varied. In a prospective study over a 5 year period, 166 patients with primary colorectal cancer had either total colonoscopy preoperatively or total colonoscopy within 6 months of surgical resection. One hundred seventy-eight cancers were detected. Synchronous cancers were found in eight patients (5 percent), and benign neoplastic polyps were demonstrated in 46 patients with single cancers (28 percent) and in 112 patients with synchronous primary cancers (38 percent). Of significance is that seven of eight (88 percent) synchronous cancers would not have been included in the standard resection for the index primary cancer. Similarly 31 of 46 neoplastic polyps (67 percent) were not in the same surgical segment as the primary cancer. Total large bowel evaluation, preferably using colonoscopy, is essential in all patients with cancer of the large bowel.