The purpose of this study is to determine whether budesonide or mesalazine is more active in the treatment of collagenous colitis.

This study will check the reproducibility of the results reported in trials with budesonide in patients with collagenous colitis. Efficacy of mesalazine was never tested in collagenous colitis by placebo-controlled trials. This trial will check the superiority of mesalazine over placebo using the common clinical symptom of collagenous colitis, which is chronic or recurrent non-bloody, watery diarrhea.

Inclusion Criteria (main):

• > 4 watery/soft stools on at least 4 days in the week prior to baseline
• > 3 stools per day on average within the last 7 days prior to baseline
• Symptoms (chronic watery diarrhea) for at least 3 months before baseline
• Complete colonoscopy within the last 12 weeks before baseline
• Histologically confirmed diagnosis of collagenous colitis

Exclusion Criteria:

• Evidence of infectious diarrhea
• Celiac disease
• Endoscopic-histologic findings, which may have caused diarrhea
• History of partial colonic resection
• Diarrhea as a result of the presence of other symptomatic organic disease of the gastrointestinal tract
• Active colorectal cancer or a history of colorectal cancer
• Severe co-morbidity substantially reducing life expectancy
• Abnormal hepatic function or liver cirrhosis (ALT, AST or AP >= 2 x ULN)
• Abnormal renal function (Cystatin C > ULN)
• Active peptic ulcer disease, local intestinal infection
• Asthma, diabetes mellitus, infection, osteoporosis, glaucoma, cataract, or cardiovascular disease if careful medical monitoring is not ensured
• Hemorrhagic diathesis