The Colitis Once Daily Asacol Study

Inclusion Criteria: Patients who meet the following criteria will be eligible for study entry: * Male and female patients aged over 18 with ulcerative colitis confirmed by histology who are in remission (no symptoms of active disease, and modified Baron sigmoidoscopic score of 0 or 1) * If female, must be (as documented in patient notes): * postmenopausal (at least 1 year without spontaneous menses), or * surgically sterile (tubal ligation or hysterectomy at least 6 months prior to enrollment), or * using acceptable contraception (e.g., oral, intramuscular, or implanted hormonal contraception) at least 3 months prior to enrollment, or * have a sexual partner with non-reversed vasectomy (with confirmed azoospermia), or * be using 1 barrier method (e.g., condom, diaphragm, spermicide, or intra-uterine device) * Patients whose ulcerative colitis has been in clinical remission for 4 weeks or longer, and who have had a symptomatic relapse within the past two years * Patients taking mesalazine, sulfasalazine or other drug containing 5-ASA for 4 weeks or longer * Patients capable of giving written informed consent Exclusion Criteria: The following patients will be excluded from the study: * Patients with Crohn's disease * Patients with symptoms of active colitis * Modified Baron sigmoidoscopy score of 2 or 3 * Patients who have used oral, enema, intravenous or suppository preparations of corticosteroids, oral or intravenous ciclosporin, mesalazine enemas or suppositories within the past four weeks * Patients taking azathioprine or 6-mercaptopurine who have altered the dose or started treatment within the past three months, (these drugs permitted in stable dose during the study) * Patients with intolerance to Asacol 400 mg or mesalazine * Women who are pregnant or lactating * Patients with known HIV infection * Patients with hepatic disease * Patients with renal impairment (creatinine above local reference range), or with positive urine dipstick test to blood or protein * Other serious medical or psychiatric illness that in the opinion of the investigator would possibly comprise the study * Patients with problem alcohol excess or drug abuse