Crohn's disease is an inflammatory bowel disease like ulcerative colitis. Distinct from ulcerative colitis, Crohn's disease may involve any portion of the alimentary tract from the mouth to the anus. Recently, the incidence of Crohn's disease has been increasing in Korea. The proportion of the colon type is smaller than that of the ileocecal type or the small-bowel type. In the colon, Crohn's disease affects mainly the right side. Relatively, the sigmoid colon is rarely involved. Small-bowel obstruction is the most common complication requiring surgery in Crohn's disease. On the contrary, an obstruction limited to the colon requiring surgery is less common in Crohn's disease. We experienced a case of a severe acute sigmoid colon obstruction with peritonitis. At first, we suspected colon cancer, but after an emergency laparotomy, we diagnosed it as Crohn's disease. Such a situation is rare in Korea, so we hope this case report may provide a good opportunity to reconsider Crohn's disease.