Collagenous colitis as a cause of chronic diarrhea. When the usual workup for chronic diarrhea fails to provide a diagnosis and the endoscopic findings are normal, alternative etiologies must be considered. This case of collagenous colitis represents such an alternative diagnosis. The patient is a 65-year-old woman who complained of abdominal cramps and watery diarrhea for an 8-month span. The key element to her diagnosis was subepithelial collagen deposits of the mucosa of the colon. Her symptoms were resolved with supportive care, diet, and diphenoxylate. Essential features and treatment of collagenous colitis are reviewed. 