Background and Study Aims: A new technique has been described which combines abdominal helical computed tomography (CT) scanning and virtual reality computer technology, known as virtual colonoscopy (VC); the reconstructed images provide a simulation of the interior of the colon as viewed by endoscopy. We compared VC with conventional colonoscopy in patients with suspected or known colonic neoplasia. Patients and Methods: A total of 38 patients, in whom there was a high likelihood of colonic polyps or cancer, underwent a noncontrast helical CT scan of the abdomen and pelvis after regular colonoscopy bowel preparation.The images were reconstructed into a VC presentation and compared with the subsequent conventional colonoscopy in a blinded manner. Results: Conventional colonoscopy identified a total of 24 polyps 5 mm or greater. VC correctly identified five of 13 polyps 5 - 9 mm in size, and ten of 11 lesions greater than or equal to 10 mm in diameter. The reasons for four missed lesions were identified as being secondary to a collapsed rectum in two patients and stool in the right colon in two patients. The sensitivity and specificity per patient of VC for lesions greater than or equal to 5 mm were 66.7 % and 75.0 % respectively, and for lesions greater than 1 cm were 90.0 % and 82.1 %, respectively. Conclusions: Virtual colonoscopy is feasible, well tolerated, and capable of detecting most lesions greater than 10 mm in diameter. This technique is continuing to be developed and warrants further evaluation as a diagnostic and screening tool in colorectal neoplasia.