Ninety percent of all colon cancers start as polyps. Therefore, the best way to prevent colon cancer is to look for precancerous polyps inside the colon and remove them before they progress to cancer. Colorectal cancer screening tests are designed to find precancerous polyps. Screening also detects cancers at an earlier stage when complete cure may be possible. Screening has been shown to prevent up to 70% of colon cancers.

Screening involves direct visualization of the inner lining of the colon to detect polyps. This can be done either by colonoscopy or sigmoidoscopy. Non-invasive techniques such as virtual colonoscopy using a CT scan can also visualize the colon lining. Other colon cancer screening tests involve stool analysis for presence of occult blood which may indirectly suggest the presence of colon polyps. Screening is recommended starting at the age of 45.