Background: Detection of colorectal tumors with computed tomography colonography (CTC) is an alternative to conventional colonoscopy (CC), and clarification of the diagnostic performance is essential for cost-effective use of both technologies. Purpose: To evaluate the diagnostic performance of CTC compared with CC. Material and Methods: 231 consecutive CTCs were performed prior to same-day scheduled CC. The radiologist and endoscopists were blinded to each other's findings. Patients underwent a polyethylene glycol bowel preparation, and were scanned in prone and supine positions using a single-detector helical CT scanner and commercially available software for image analysis. Findings were validated (matched) in an unblinded comparison with video-recordings of the CCs and re-CCs in cases of doubt. Results: For patients with polyps ⩾5 mm and ⩾10 mm, the sensitivity was 69% (95% CI 58–80%) and 81% (68–94%), and the specificity was 91% (84–98%) and 98% (93–100%), respectively. For detection of polyps ⩾5 mm and ⩾10 mm, the sensitivity was 66% (57–75%) and 77% (65–89%). A flat, elevated low-grade carcinoma was missed by CTC. One cancer relapse was missed by CC, and a cecal cancer was missed by an incomplete CC and follow-up double-contrast barium enema. Conclusion: CC was superior to CTC and should remain first choice for the diagnosis of colorectal polyps. However, for diagnosis of lesions ⩾10 mm, CTC and CC should be considered as complementary methods.