BACKGROUND: Several studies examined the rate of colorectal cancer (CRC) developed during colonoscopy surveillance after CRC resection (ie, CRC [mCRC]), yet the underlying etiology is unclear. OBJECTIVE: To rate and likely etiology of mCRCs. DESIGN: Population-based, multicenter Review of clinical and histopathologic records, including data of the pathology database and The Netherlands Cancer Registry. SETTING: databases reviewed at 3 hospitals in South-Limburg, The Netherlands. Total CRC population diagnosed in South-Limburg from January 2001 to 2010. INTERVENTIONS: Colonoscopy. MAIN OUTCOME MEASUREMENTS: We defined as a second primary CRC, diagnosed >6 months after the primary CRC. By modified algorithm to ascribe likely etiology, we classified the mCRCs cancers caused by non-compliance with surveillance recommendations, examination, incomplete resection of precursor lesions (CRC in same previous advanced adenoma), missed lesions, or newly developed cancers. We included a total of 5157 patients with CRC, of whom 93 (1.8%) had were diagnosed on an average of 81 months (range 7-356 months) after the CRC diagnosis. Of all mCRCs, 43.0% were attributable to non-compliance surveillance advice, 43.0% to missed lesions, 5.4% to incompletely lesions, 5.4% to newly developed cancers, and 3.2% to inadequate Age-adjusted and sex-adjusted logistic regression analyses showed that significantly smaller in size (odds ratio [OR] 0.8; 95% confidence 0.7-0.9) and more often poorly differentiated (OR 1.7; 95% CI, 1.0-2.8) solitary CRCs. LIMITATIONS: Retrospective evaluation of clinical data. CONCLUSION: In this study, 1.8% of all patients with CRC developed vast majority were attributable to missed lesions or non-compliance with surveillance advice. Our findings underscore the importance of high- colonoscopy to maximize the benefit of post-CRC surveillance.