Colorectal cancer is cancer of the colon (large intestine) or rectum. Second colorectal cancer can occur in patients after treatment for first colon cancer. For this reason, patients typically have surveillance with periodic colonoscopy after treatment for colon cancer. Colonoscopy involves looking into the rectum and colon through a tube-shaped instrument for precancerous abnormalities or tumors. Current recommendations suggest that patients should have colonoscopy 1 year after treatment; then, if the results of the first colonoscopy are normal, they should have colonoscopy every 3 to 5 years. However, it is not known whether this follow-up is adequate because limited information is available about the frequency and timing of second cancer.