The features of collagenous colitis are briefly summarized on the basis of 24 own patients and a review of the literature. Collagenous colitis is a rare diarrheal disease first described in 1976. The hallmark of this syndrome consists in a thickened layer of subepithelial collagen in the entire colon. Clinically patients present with longstanding, watery diarrhea. Laboratory tests, radiology and colonoscopy are all normal. Therefore the diagnosis is only established, if an alert colonoscopist takes biopsies out of an endoscopically normal appearing mucosa. There is no specific treatment, but patients usually respond to symptomatic therapy with antidiarrheal drugs. Microscopic colitis may represent an early stage of collagenous colitis.