BACKGROUND & AIMS: Collagenous colitis is an idiopathic microscopic colitis characterized by chronic watery diarrhea, a typical subepithelial collagen layer, and lymphoplasmacellular infiltration. We investigated the effect of budesonide on symptoms and histology in patients with collagenous colitis in a randomized, double-blind, placebo-controlled multicenter trial. METHODS: Patients with chronic diarrhea and histologically proven collagenous colitis were randomized to receive either oral budesonide (Entocort capsules; AstraZeneca, Sodertalje, Sweden) 9 mg/day for 6 weeks or placebo. Complete colonoscopy was performed before and after treatment. Histopathology was assessed by a single pathologist blinded to the patients' treatment. Clinical symptoms were assessed by standardized questionnaires. RESULTS: Fifty-one patients were randomized; 45 patients were available for per protocol analysis. The rate of clinical remission was significantly higher (P < 0.001) in the budesonide group than in the placebo group (per protocol 86.9% vs. 13.6%, respectively; intention-to-treat 76.9% vs. 12.0%, respectively). Histologic improvement was observed in 14 patients of the budesonide group (60.9%) and in 1 patient of the placebo group (4.5%; P < 0.001). Two patients in the budesonide group (7.7%) and 1 patient in the placebo group (4.0%) discontinued treatment prematurely because of side effects. CONCLUSIONS: Oral budesonide (Entocort capsules) is an effective and safe treatment modality for patients with collagenous colitis. Long-term follow-up of these patients is necessary to investigate whether clinical and histologic remission is sustained.