Collagenous and lymphocytic colitis are newly recognized causes of chronic watery diarrhea that typically affect middle-aged patients. Although endoscopic studies are normal, inflammatory changes and (in the case of collagenous colitis) collagen deposition occur histologically in the colonic mucosa. The pathogenesis of these disorders remains a mystery, but the possible causes are intriguing. Patients may experience spontaneous remissions and relapses, but treatment with sulfasalazine or prednisone is usually effective for patients with distressing symptoms.