Eleven patients with so-called collagenous colitis are described and the literature reviewed. The disease presents with persisting watery diarrhoea in middle-aged subjects, predominantly women. The fairly uniform clinical features of abdominal discomfort are suggestive of the irritable bowel syndrome. The morphological changes in colorectal biopsy specimens are diagnostic, showing an excessive intercryptal subepithelial collagen deposition throughout the large bowel. Associated hyperplasia, degeneration, and desquamation are seen in the intercryptal epithelial cells and a mild inflammatory response in the lamina propria. A comparable collagenization has not been demonstrated in other disorders, but otherwise the changes demonstrated histologically and ultrastructurally are of a quantitative nature. Collagenous colitis is unrelated to other diseases and the cause unknown. It has either a benign, continuous course or exacerbations and remissions. Loperamide relieved diarrhoea in five of six patients. The collagen deposition seems to be slowly progressive, but clinical and histopathological resolution may occasionally be seen.