OBJECTIVE: The quality of colonoscopy is key for ensuring protection colorectal cancer (CRC). We therefore aimed to elucidate the aetiology postcolonoscopy CRCs (PCCRCs), and especially to identify preventable METHODS: We conducted a population-based study of all patients diagnosed in South-Limburg from 2001 to 2010 using colonoscopy and histopathology and data from the Netherlands Cancer Registry. PCCRCs were defined as diagnosed within 5 years after an index colonoscopy. According to were categorised into proximal or distal from the splenic flexure and, to macroscopic aspect, into flat or protruded. Aetiological factors for were subdivided into procedure-related (missed lesions, inadequate examination/surveillance, incomplete resection) and biology-related (new cancers). RESULTS: We included a total of 5107 patients with CRC, of (2.9% of all patients, mean age 72.8 years, 55.1% men) had PCCRCs average 26 months after an index colonoscopy. Logistic regression adjusted for age and gender, showed that PCCRCs were significantly more proximally located (OR 3.92, 95% CI 2.71 to 5.69), smaller in size (OR CI 0.70 to 0.87) and more often flat (OR 1.70, 95% CI 1.18 to 2.43) than prevalent CRCs. Of the PCCRCs, 57.8% were attributed to missed lesions, inadequate examination/surveillance and 8.8% to incomplete resection, were newly developed cancers. CONCLUSIONS: In our experience, 86.4% of could be explained by procedural factors, especially missed lesions. improvements in performance of colonoscopy, with special attention to detection and resection of proximally located flat precursors, have the to prevent PCCRCs.