We report here a case of ulcerative colitis (UC) with a leaked colojejunal fistula. The patient was a 25-year-old woman with a history of UC for 14 years and with a colojejunal fistula between the decending colon and the jejunum. She was admitted to our department for a sudden attack of severe abdominal pain. On physical examination, the abdomen was distended and generally tender with signs of peritoneal irritation. Abdominal computed tomography (CT) revealed ascites standing in the abdominal cavity and fluid collection around the descending colon. Accordingly, she was diagnosed as having a panperitonitis. An emergency laparotomy revealed advanced UC with the leaked colojejunal fistula. She underwent a subtotal colectomy with ileostomy and rectostomy and a partial resection of the jejunum including the fistula. The presence of an entero-enteric fistula, of which a colojejunal fistula is one type, is characteristic in Crohn's disease but is the rarest complication in UC, and only 11 cases including our case have been reported in the world. This seems to be the fistula in UC.