Collagenous colitis is a clinicopathologic syndrome with chronic watery diarrhea, diffuse colitis with surface epithelial injury, and a distinctive collagen band beneath the surface epithelium especially in the proximal colon. The cases of seven patients (including six middle-aged women) with chronic, watery, noninfectious diarrhea were studied. Roentgenographic and endoscopic findings were not diagnostic. Two patients had rectal mucosal inflammation but sparing of the distal colon from subepithelial collagen. Other findings included thyroid disease (four patients), urethral fibrosis (three), elevated erythrocyte sedimentation rate (six), and eosinophila (three). The colon was thought to be the main source of diarrheal fluid, but bile salt malabsorption, steatorrhea, and net small-bowel secretion were additive factors in some patients. With antiinflammatory treatment the diarrhea abated, the surface epithelial injury decreased, and the subepithelial collagen resolved (two patients), but lamina propria inflammation persisted. Collagenous colitis seems to be a chronic systemic, and perhaps autoimmune, disorder.