Phlegmasia cerulea dolens, characterized by the triad of limb swelling, cyanosis, and acute ischemic pain, usually arises because of acute massive thrombosis of major deep, collateral, and superficial veins of an extremity. We report a patient with an atypically presenting ruptured aortic aneurysm masquerading as phlegmasia cerulea dolens. A 68-year-old man with a history of hypertension, intermittent claudication, and smoking presented with asthenia, macrohematuria, and mild back pain, as well as edema and blue mottling of the lower limbs and abdomen for 24 hours. The abdomen was nontender without a pulsatile mass or murmur. Computed tomography showed an 85-mm abdominal aortic aneurysm ruptured into the inferior vena cava. Phlegmasia cerulea dolens-like symptoms were explained by compromised venous outflow from the lower limbs because of an aortocaval fistula. The patient died of uncontrollable hemorrhage during emergent surgical repair. Three percent to 6% of patients with abdominal aortic aneurysm rupture present an aortocaval fistula, which can be suspected by the triad of abdominal or lower back pain, pulsatile abdominal mass, and machinery-type abdominal murmur. Left untreated, this condition rapidly leads to death from congestive heart failure. Only one third of patients are diagnosed before surgery because a fistula is often not suspected until unexplained massive hemorrhage occurs during the operation. Operative mortality is extremely high (34%). We propose that the clinical syndrome presented by our patient, a "blue fistula" frequently associated with aortocaval fistulae, be called phlegmasia cerulea non dolens, by analogy to Grégoire's "blue phlebitis."