Phlegmasia cerulea dolens is a rare form of massive proximal venous thrombosis of the lower extremities associated with a high degree of morbidity, presenting with sudden severe lower extremity pain with edema and cyanosis. Venous gangrene, venous congestion with massive fluid sequestration leading to circulatory collapse, and shock causing death can ensue if not promptly diagnosed and treated. We present a clinical image of a 55-year-old man with newly diagnosed cholangiocarcinoma who presented with worsening left lower extremity pain. We found painful swelling with purple discoloration extending up to the ankle joint. Distal arterial pulses were palpable. Doppler ultrasound revealed extensive thrombosis in all segments of the deep vein bilaterally except the right iliac segment. Anticoagulation with heparin was initiated. The patient underwent an emergency left iliac thrombectomy for salvage of the limb and placement of a prophylactic inferior vena cava filter. The patient's symptoms resolved.