A 76-year-old man who had been hospitalized for medical treatment of a liver abscess and sepsis since June 15, 2002 vomited out a large quantity of blood on June 25, but, he had neither abdominal pain nor abdominal manifestations. Abdominal CT scan showed a diffuse retroperitoneal abscess with emphysema extending from the anterior part of the right kidney to the 2nd portion of the duodenum. Retroperitoneal perforation of the duodenum was diagnosed, and an emergency laparotomy was performed. We suspected the presence of a duodenal diverticulum, but we found a huge abscess cavity in the retroperitoneum, instead of diverticulum, which extended from the anterior surface of the right kidney to the 2nd portion of the duodenum. There were remarkable inflammatory changes in the abscess cavity. After opening the cavity and Kocher mobilization, a perforation, 2±1.5cm in size, was found in the 2nd portion of the duodenum. Because the perforating defect was large with severely inflammatory change, the simple closure of the defect was thought to be difficult and dangerous. For this reason, an omentoplasty for the duodenal defect was employed. Furthermore, a gastro-jejunostomy was added in consideration of postoperative duodenal stenosis. The postoperative course was uneventful, though the patient was in poor condition due to severe diabetes mellitus.