Mette Kielsholm Thomsen,1 Sisse Helle Njor,1 Morten Rasmussen,2 Dorte Linnemann,3 Berit Andersen,4 Gunnar Baatrup,5,6 Lennart Jan Friis-Hansen,7 Jens Christian Riis Jørgensen,8 Ellen Margrethe Mikkelsen1 1Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, 2Department of Digestive Diseases K, Bispebjerg Hospital, Copenhagen, 3Department of Pathology, Herlev and Gentofte Hospital, Herlev, 4Department of Public Health Programs, Randers Regional Hospital, Randers, 5Department of Surgery, Odense University Hospital, 6Department of Clinical Science, University of Southern Denmark, Odense, 7Center for Genomic Medicine, Rigshospitalet, University of Copenhagen, Copenhagen, 8Department of Colorectal Cancer Surgery, Vejle Hospital, Vejle, Denmark Background: In Denmark, a nationwide screening program for colorectal cancer was implemented in March 2014. Along with this, a clinical database for program monitoring and research purposes was established. Objective: The aim of this study was to estimate the agreement and validity of diagnosis and procedure codes in the Danish Colorectal Cancer Screening Database (DCCSD). Methods: All individuals with a positive immunochemical fecal occult blood test (iFOBT) result who were invited to screening in the first 3 months since program initiation were identified. From these, a sample of 150 individuals was selected using stratified random sampling by age, gender and region of residence. Data from the DCCSD were compared with data from hospital records, which were used as the reference. Agreement, sensitivity, specificity and positive and negative predictive values were estimated for categories of codes “clean colon”, “colonoscopy performed”, “overall completeness of colonoscopy”, “incomplete colonoscopy”, “polypectomy”, “tumor tissue left behind”, “number of polyps”, “lost polyps”, “risk group of polyps” and “colorectal cancer and polyps/benign tumor”. Results: Hospital records were available for 136 individuals. Agreement was highest for “colorectal cancer” (97.1%) and lowest for “lost polyps” (88.2%). Sensitivity varied between moderate and high, with 60.0% for “incomplete colonoscopy” and 98.5% for “colonoscopy performed”. Specificity was 92.7% or above, except for the categories “colonoscopy performed” and “overall completeness of colonoscopy”, where the specificity was low; however, the estimates were imprecise. Conclusion: A high level of agreement between categories of codes in DCCSD and hospital records indicates that DCCSD reflects the hospital records well. Further, the validity of the categories of codes varied from moderate to high. Thus, the DCCSD may be a valuable data source for future research on colorectal cancer screening. Keywords: colorectal cancer screening, clinical database, data validity