Routine preoperative colonoscopy has been recommended for those patients with the diagnosis of colorectal cancer to identify synchronous polyps and/or cancers which might otherwise be undetected on barium enema or at the time of operation. It has been suggested that this approach may alter surgical therapy or follow-up. The charts of 98 patients who underwent preoperative colonoscopy solely for the purpose of detecting additional polyps or cancers prior to open surgical resection for colorectal cancer were retrospectively reviewed. All patients had biopsy or barium-enema evidence of colorectal cancer. Patients with familial polyposis or chronic ulcerative colitis were excluded. Synchronous cancers were detected in 7.1 percent of patients, and synchronous neoplastic polyps in 29 percent. Forty-three percent of synchronous cancers and 73 percent of synchronous neoplastic polyps would not have been included in the standard surgical resection for the index cancer if the additional information provided by colonoscopy had not been available. Surgical treatment and/or follow-up were altered in 33 percent of patients as a consequence of the colonoscopic evaluation. Colonoscopy prior to surgery for colorectal carcinoma is highly desirable and may potentially improve long-term survival.