Provided that the bowel is carefully cleansed, virtual colonoscopy permits a safe, reliable and painfree diagnosis of the large bowel. It requires distension of the bowel with ambient air. Contraindications include pregnancy, acute inflammation of the bowel and recent surgery. Tumors and large polyps (10 mm and more) can be diagnosed with certainty, while polyps measuring 5 mm and more are detected with a high degree of sensitivity. It goes without saying that virtual colonoscopy does not allow for the taking of biopsy material or the performance of therapeutic measures, so that positive or suspicious findings must be clarified by conventional colonoscopy. In principle, virtual colonoscopy is also suitable for use as a screening procedure. Whether the radiation exposure associated with the conventional examination protocol can be considered acceptable for screening purposes is questionable. However, using mathematical means, the radiation dose can be reduced to one-tenth of the usual dose.