Collagenous colitis is a clinicopathological syndrome characterized by: (1) chronic watery diarrhea and crampy abdominal pain and (2) thickened subepithelial collagen band and increased intraepithelial lymphocytes on histology. The mucosa generally appears endoscopically normal, although some nonspecific abnormalities such as patchy erythema, an abnormal vascular pattern or erythema may be found in upto one third of cases. Collagenous colitis is generally regarded as a benign disease and serious complication are uncommon. The frequency of colonic perforation in collagenous coliltis is unknown, but is probably very rare. Only 16 patients, excluding our cases, have so far been reported. We report a case of a 80-year-old woman with collagenous colitis who presented with colonic perforation two days after the colonoscopy. (Intest Res 2007;5:77-80)