Collagenous colitis, a cause of watery diarrhea characterized by a distinctive band of collagen under the surface epithelium of the colon, has been recognized with increasing frequency in recent years. The pathophysiology of collagenous colitis remains obscure. The thickening of the subepithelial collagen layer may be a response to chronic inflammation or a local abnormality of collagen synthesis. The precise mechanism of the diarrhea in collagenous colitis is also unclear, and it has not been possible to link the diarrhea directly to the excess collagen deposition. The relationship between collagenous colitis and lymphocytic colitis, another type of microscopic colitis, remains to be defined; elucidating the relationship between the two disorders may provide clues to the pathophysiology of both.