The authors report a case of an extended retroperitoneal abscess that developed secondarily after a duodenal diverticulum perforation. The diagnosis was established preoperatively endoscopically and by abdominal CT scan. The patient was treated surgically. The authors performed a subtotal gastrectomy and reconstruction with antecolic Billroth II anastomosis to bypass the inflamed region, as well as simultaneous drainage of the retroperitoneal abscess. The postoperative course of the patient was uncomplicated. The authors suggest that this method may be an acceptable approach in the management of similar cases, in which conservative or endoscopic means have failed.