The purpose of this study is to investigate the efficacy and safety of 4-week double-blind treatment with PENTASA enema compared with placebo enema in Chinese patients with mild to moderate active left-sided ulcerative colitis, followed by a maximal 28-week open-label extension phase with PENTASA enema and/or PENTASA tablets.

Inclusion Criteria:

• Newly diagnosed or recurrent mild to moderate left-sided active ulcerative colitis
• Mayo score of at least 4 but not greater than 10 points and a score of ≥ 2 for colonoscopy
• Extent of colonic involvement and endoscopy subscore of Mayo score as confirmed by colonoscopy
• The patient is compliant with Patient Daily Diary by having adequately responded to the questions on ≥5 of the most recent full 7 days before the Randomization Visit
• Negative stool test at screening to rule out parasites and bacterial pathogens

Exclusion Criteria:

• Patients receiving > 2g/day of oral 5-aminosalicylic acid (5-ASA) products, or receiving topical rectal 5-ASA > 3g/week, within 7 days prior to screening
• Severe/fulminant ulcerative colitis or toxic dilatation of the colon
• Prior bowel resection surgery
• Patients allergic to 5-ASA and derivative, any of the excipients, aspirin, or salicylates
• Patients with one or more of the diseases: bacillary dysentery, amebic dysentery, chronic schistosomiasis, intestinal tuberculosis and Crohn's disease
• Patients who are unable to fill in the Patient Daily Diary or follow data-capturing procedures