This is a multi-centre, randomised, controlled, investigator blinded study. The randomisation will be done centrally. The patients will be treated for 1 year, with clinical and laboratory assessments at 0, 4, 8 and 12 months.

Endoscopic examination is at enrollment and on completion of the study (at relapse or after 12 months).

Number of Subjects (Planned and Analysed):

• 360 patients for demonstration of non-inferiority between once daily and twice daily;
• 326 to be analysed in per-protocol (PP) analyses; and
• 360 in intention-to-treat (ITT) analyses.

Inclusion Criteria:

1. Patients have an established diagnosis of ulcerative colitis and are in clinical remission with an UC-DAI < 2 at enrollment
2. Extension of the disease > 15 cm distance from anal verge
3. Patients who have had a clinical relapse within the past year. Clinical relapse is defined as activity of the disease for which maintenance therapy had to be adjusted.
4. Patients on oral mesalazine maintenance therapy ≤ 2.5 grams per day
5. 18 years or older
6. Signed informed consent

Exclusion Criteria:

1. Patients with evidence of other forms of inflammatory bowel disease, idiopathic proctitis or infectious disease
2. Patients allergic to acetylsalicylic acid and other salicylate derivates aspirin or salicylates derivatives
3. Patients who used mesalazine > 2.5 grams orally in the previous month,
4. Patients who used rectal mesalazine > 3 grams per week in the previous month
5. Use of corticosteroids (oral and/or rectal routes) within the last month
6. Intake of immunosuppressants within the last 3 months
7. Patients with (known) significant hepatic (up to 2 x upper limit of normal) or (known) renal function abnormalities, to 1.5 x upper limit of normal values
8. Patients with history or physical examination findings indicative of active alcohol or drug abuse
9. Patients with a history of disease, including mental/emotional disorders, that would interfere with their participation in the study
10. Women who are pregnant or nursing (non-menopausal women who are sexually active and do not use effective contraceptives, as judged by the investigator, must have a negative pregnancy test)
11. Patients who participated in another clinical study in the last 3 months
12. Patients who were previously participating in this study
13. Patients with any other disease that may influence the study assessment, such as malignant disease, etc.
14. Patients who are unable to comply with any requirements of the protocol
15. Patients who are unable to write or read local language.