Collagenous colitis is a newly described entity that clinically manifests itself as watery diarrhea of long-standing duration. The main histopathologic characteristic is the presence of a collagen band immediately beneath the colonic surface epithelium. Ultrastructurally, the collagen is deposited beneath the basement membrane, which is intact. Pathogenetically, an aberrant function of the pericryptal fibroblastic sheath may be involved.