SummaryThree cases of collagenous colitis are described. This is a hitherto infrequently recognized disease that presents as watery diarrhea associated with abdominal cramping pain and/or vague discomfort. It is usually a chronic condition but spontaneous remissions and exacerbations have been observed. Incidence peaks in the fifth decade of life and there is a marked female predominance.The diagnosis is established on the basis of histologic findings: the presence of a broad subepithelial collagenous layer in a macroscopically normal colonic mucosa.The etiology is not known and the pathophysiology is poorly understood. An important role may perhaps be attributed to local PGE: production in the colon.Treatment has so far been empirical. Collagenous colitis might be more frequently diagnosed if colonic biopsies were performed as a matter of routine in all patients with functional bowel disease.