A 78-year-old man was admitted to our hospital for lower abdominal pain. Intermittent melena was noted during anticoagulant therapy for acute arterial obstruction of the left lower extremity. A barium contrast radiograph revealed a cavity communicating with the jejunal lumen, and abdominal CT showed an air-containing tumor about 10cm in diameter. The pain on admission was likely to have been attributable to infection inside the tumor, because antibiotic administration relieved the pain. Subsequent development of systemic inflammatory responses and physical signs of peritoneal irritation were considered indications for an emergency operation. Laparotomy revealed purulent ascites and a jejunal tumor exhibiting extramural growth. Massive necrosis was observed on the cut surface of the resected tumor, which measured 11×12×9cm in size. In addition to the findings on histological examination after HE staining, immunostaining for c-kit was positive. As a result, the tumor was diagnosed as a GIST. We suggest that peritonitis is one of complications of GIST.