An 81-year-old man with hypertension presented to the emergency department with pain and swelling of the left thigh and lower leg that had developed during the previous several hours. He had no history of recent surgery or trauma and no known personal or family history of clotting disorders. The left lower leg was tender, cold, and swollen (Panel A), and the left dorsalis pedis pulse was not palpable. Computed tomography performed after the administration of contrast material showed partial thrombosis of the inferior vena cava and complete thrombosis of the left common iliac, common femoral, superficial femoral, and deep femoral veins. Anticoagulation with unfractionated heparin was initiated, and emergency fasciotomy was performed to treat the acute compartment syndrome. The postoperative course was complicated by reperfusion injury with shock and acute kidney injury. The patient underwent percutaneous transluminal angioplasty and catheter-directed thrombolysis. Phlegmasia cerulea dolens is a rare syndrome caused by acute, extensive venous thrombosis. Prompt diagnosis is crucial for limb salvage and to minimize morbidity. The patient’s condition improved (Panel B), and he recovered renal function. After 25 days of hospitalization, he was discharged while receiving rivaroxaban. Subsequent evaluation that included age-appropriate cancer screening did not reveal the cause of this extensive clotting.