Most perforations of the duodenum occur into the free abdominal cavity and produce widespread contamination of the peritoneum. The signs and symptoms are well known and the diagnosis is promptly established. Early treatment has resulted in a surprisingly low mortality. In retroperitoneal perforations the reverse is true. The same area is involved and the same duodenal contents escape through the rent in the wall. Nevertheless, the mortality remains at 90 per cent. While the technical difficulties of repair and drainage are admittedly a factor, this high mortality is largely due to errors in diagnosis and procrastination in treatment. Any improvement in the accuracy and promptness of diagnosis should surely reflect a reduction in the mortality. Etiology Although many retroperitoneal perforations of the duodenum occur as complications of peptic ulcer, the majority are due to trauma. In spite of its well protected anatomical location, about 10 per cent of all traumatic ruptures of the gastro-intestinal tract occur in this part of the bowel, one-third of them in its retroperitoneal portion. Occasionally they are multiple and perforate intraperitoneally and retroperitoneally at the same time. Most of these injuries involving the duodenum are of the blunt type; they are believed to occur in one of three ways, namely, crushing of the bowel between the blunt force and the spine, bursting of a distended loop the ends of which are momentarily closed, and tearing at a region of fixed ligamentous attachment. Most of the cases reported have been due to such injuries as kicks or blows in the abdomen, crushing between heavy objects, or being run over by a vehicle. Compound injuries, in which there is a communication with the outside, will not be considered. Clinical Features The clinical features are especially important. Unless the attending physician bears in mind the possibility of a retroperitoneal perforation, the hope of an early diagnosis is lost. This is especially true in traumatic cases. It is a striking observation that comparatively few patients are greatly inconvenienced by the original trauma. Only a few vomit immediately, and the pain is in no sense severe. Many are able to walk following the injury. Few are unconscious. It is only upon the advent of the effects of the extravasation that the symptoms become marked. This is the reverse of intraperitoneal perforations. Rigidity is not a prominent symptom and when it develops, usually after several hours, it means that communication with the free abdominal cavity has occurred and peritonitis is developing. Laparotomy at this stage will reveal the hopelessness of the situation. The extravasated material, consisting of blood, bile, pancreatic juice, and other duodenal contents, is doubtless infected by this time and will have produced retroperitoneal cellulitis and necrosis. Peritonitis produced by such material cannot be expected to respond to treatment.