SummarySix patients with collagenous colitis are described and the literature is reviewed. Collagenous colitis is not a rare disease. Its most prominent feature is profuse watery diarrhoea. Although the endoscopic picture of the mucosa is usually normal, there is a microscopic thickening of the (subepithelial) basement membrane. For diagnostic purposes, multiple colon biopsies are warranted in patients with chronic or relapsing diarrhoea. There is no established therapy for this disease.