Collagenous colitis is a rare condition characterised by watery diarrhoea. It is thought to be caused by a thick collagenous membrane found immediately beneath the colonic surface epithelium. We have studied specimens from 457 patients with a wide variety of large bowel diseases in order to determine the frequency, nature, and clinical correlations of the thick membranes. The normal membrane measured up to 3 microns. In 19 (4%) of the patients studied there was significant thickening (greater than 10 mu). In 12 patients the thickening ranged from 10-15 mu, and six of this group had diarrhoea. In seven, the membrane exceeded 15 mu, and six patients had diarrhoea. There was no correlation between the nature of the disease process, bleeding, pain, constipation or previous medication. It did not vary with the age, nor at different sites of the colon. We concluded that no case in this series represented an example of true collagenous colitis but that significant thickening of the membrane occurs in other disease conditions and that a thick membrane, no matter how it arises, is causally associated with profuse watery diarrhoea.