In four patients with ruptured abdominal aortic aneurysms increased intra-abdominal pressure developed after repair. It was manifested by increased ventilatory pressure, increased central venous pressure, and decreased urinary output associated with massive abdominal distension not due to bleeding. This set of findings constitutes an intra-abdominal compartment syndrome caused by massive interstitial and retroperitoneal swelling. The purpose of this report is to establish criteria for this syndrome and suggest a method of treatment. The syndrome developed within 24 hours; in one patient within 5 hours postoperatively. All four patients received more than 25 liters of fluid resuscitation (electrolyte and blood) during and within 16 hours after operation and had massive abdominal distension. Decompressive laparotomies were performed in the Intensive Care Unit with placement of Marlex (Bard Corp., Billerica, MA) mesh. In two additional patients, at the completion of the aneurysmectomy the abdominal incision was left open with interposition Marlex mesh. Opening the abdominal incision was associated with dramatic improvements in central venous pressure, urinary output, ventilatory pressure, arterial carbon dioxide tension, and oxygenation. The authors conclude that some patients with ruptured abdominal aortic aneurysm do not tolerate the closure of the abdominal wall, as manifested by increased ventilatory pressures, decreased oxygenation, and decreased urinary output. Opening the abdominal wound or delayed closure may reverse the oliguria and improve oxygenation. Recognition and treatment of this condition by opening the abdominal wound or delayed closure may affect outcome in some cases.