A 73-year-old woman treated with steroids for exacerbation of ulcerative colitis visited our hospital with a chief complaint of watery diarrhea. Despite conservative treatment for acute gastroenteritis, her general condition deteriorated rapidly. On examination, she was detected with pulmonary embolism and acute cholecystitis on the 5th hospital day. We administered anticoagulant therapy and antimicrobial treatment; however, her general condition deteriorated rapidly, and she died on the 19th hospital day. Autopsy findings showed pulmonary embolism, deep vein thrombosis, megacolon and intestinal perforation in the right side of the colon, and ileal backwash ileitis. We suspected that she developed multiple thrombosis caused by exacerbation of ulcerative colitis, and died of intestinal perforation and panperitonitis. Despite a decrease in the number of operations because of the advances in medical treatment, surgical intervention without delay is crucial in cases showing acute exacerbation.