BACKGROUND Collagenous colitis is an inflammatory disease characterized by hyperplasia of the collagen band beneath the colonic mucous membrane. Chronic diarrhea is a characteristic clinical symptom. The disease is often diagnosed accidentally on colonoscopy for chronic diarrhea, and patients without chronic diarrhea have few chances to suspect the disease. METHODS The patient was a 75-year-old woman. The chief complaint was sudden upper abdominal pain and vomiting. There were no important findings regarding the consumed food or bowel habits (no diarrhea). Computed tomography revealed wall thickness and a small amount of free air around the descending colon. An emergency laparotomy was performed with the diagnosis of spontaneous colonic perforation. Intra-operative findings revealed a longitudinal ulcer and micro-perforation to the mesenterial side at the descending colon. Pathological findings revealed subepithelial collagenous band in the submucosal background of the ulcer, and which was diagnosed as collagenous colitis. CONCLUSIONS Intestinal perforation in collagenous colitis is extremely rare. It was considered that perforation was caused by a transient increase in intestinal pressure in the background of collagenous colitis. Further, to the best of our knowledge, this is the first report of a critical case which presented without the characteristic symptom of chronic diarrhea. CONCLUSIONS We report a rare case of colonic perforation of the collagenous colitis.