ABSTRACT Background - Computed tomography colonography (CTC) is about to become a valid alternative to conventional colonoscopy in detection of polyps and colorectal cancer (CRC). This is a retrospective study comparing CTC to colonoscopy as a method used for diagnostics at Oslo University Hospital, Ullevål. The study is presented as a part of the medical degree program at University of Oslo. Aim of the study - The study looks at aspects of how same-day colonoscopy has performed in a day-to-day clinical setting in relation to quality assurance. The detection rate for CTC was compared using optical colonoscopy as a gold standard. Material and methods - Inclusion criteria for relevant patients were defined as all patients who attended CTC with a following colonoscopy the same day in the period 1st June 2005 to 30th June 2013. Patients without significant findings after CTC or with incomplete bowel preparation were excluded. In total 58 patients were included in the study. Results – Looking at a sub selection of patients only with optimal colonoscopy, we estimated a CTC per polyp PPV = 70,6%, and an amount of false positive lesions 20%. Adjusted for patients referred because of wall thickening ≥10 mm, due to diverticulitis or post surgical controls, we estimated a per polyp PPV = 77,4%. Colonoscopy was not able to verify polyps found after CTC in 16% of all lesions. CTC missed 3 polyps ≥10 mm, but none out of 3 cancerous lesions. Conclusions – CTC at OUS, Ullevål, has in this study, not proved to have the same diagnostic qualities as compared to colonoscopy for detection of polyps and CRC. However, because a broad variety of data bias applies, this conclusion is not applicable for today s practice. The study, on the other hand, led to greater understanding of everyday routines, and suggestions of quality improvements are now implemented in the same-day colonoscopy option offered at the hospital.