This paper re-evaluates how effectively regular colonoscopy protects people with Lynch syndrome, a common inherited cause of bowel cancer. While colonoscopy saves lives by detecting cancers at an early stage, large contemporary studies show it does not reliably prevent cancers from developing. This is because Lynch syndrome cancers can grow rapidly or arise without visible polyps that can be removed. The review explains why earlier studies likely overestimated cancer prevention, highlighting important differences between the different Lynch syndrome genes, and shows that surveillance mainly reduces death rather than cancer incidence. It also identifies priorities for improving care, including higher-quality colonoscopy, better adherence, and complementary strategies such as aspirin, biomarkers, and AI-assisted detection. (https://jmg.bmj.com/content/early/2026/03/13/jmg-2025-111311)