 Patients will undergo two colonoscopies each, with an interval of three months between procedures. This minimum time interval is chosen in order to allow for the healing of the mucosa on sampled areas and thus prevent recognition of biopsy sites. The first procedure will be randomly allocated to be either conventional white light endoscopy coupled with targeted and random biopsies or high-resolution endoscopy with FICE system and magnification. Randomisation will be achieved prior to the first endoscopy by means of sealed envelopes. In each recruitment centre, one of the two endoscopists with experience in endoscopic surveillance and treatment of ulcerative colitis will be assigned to carry out the first procedure. The second procedure will automatically be scheduled with the second endoscopist, who will be blinded as to the clinical and histological findings of the first investigation. 