We herein report a case of hemophagocytic syndrome that developed in a 25-yr-old man with fulminant ulcerative colitis and presumed acute pancreatitis. Physical examination on admission showed a chronically ill, delirious patient with an upper abdominal mass. Peripheral blood showed progressive pancytopenia and bone marrow aspirate smears revealed hypocellular bone marrow with an increase of histiocytes showing prominent hemophagocytosis. Plain abdominal radiography revealed toxic megacolon. Both ultrasound and computed tomography showed the enlargement of the pancreas, thus indicating presumed acute pancreatitis. No apparent neoplasms or viral or bacterial infections, which are normally reported to be the cause of hemophagocytic syndrome, were detected. The patient was successfully treated with high doses of prednisolone and gamma-globulin.