Population-based screening programs for colorectal cancer rely heavily on the performance of colonoscopy as either the initial examination or as the follow-up to a positive screening by virtual colonography, double-contrast [...], Background: The frequency of polypectomy is an important indicator of quality assurance for population-based colorectal cancer screening programs. Although administrative databases of physician claims provide population-level data on the performance of polypectomy, the accuracy of the procedure codes has not been examined. We determined the level of agreement between physician claims for polypectomy and documentation of the procedure in endoscopy reports. Methods: We conducted a retrospective cohort study involving patients aged 50-80 years who underwent colonoscopy at seven study sites in Montreal, Que., between January and March 2007. We obtained data on physician claims for polypectomy from the Regie de l'Assurance Maladie du Quebec (RAMQ) database. We evaluated the accuracy of the RAMQ data against information in the endoscopy reports. Results: We collected data on 689 patients who underwent colonoscopy during the study period. The sensitivity of physician claims for polypectomy in the administrative database was 84.7% (95% confidence interval [CI] 78.6%-89.4%), the specificity was 99.0% (95% CI 97.5%-99.6%), concordance was 95.1% (95% CI 93.1%-96.5%), and the kappa value was 0.87 (95% CI 0.83-0.91). Interpretation: Despite providing a reasonably accurate estimate of the frequency of polypectomy, physician claims underestimated the number of procedures performed by more than 15%. Such differences could affect conclusions regarding quality assurance if used to evaluate population-based screening programs for colorectal cancer. Even when a high level of accuracy is anticipated, validating physician claims data from administrative databases is recommended.