Unsuccessfully treated iliofemoral venous thrombosis can result in pulmonary embolism, phlegmasia cerulea dolens, and post-thrombotic syndrome. Phlegmasia cerulea dolens is characterized by tense swelling of the lower extremity with tenderness of the thigh over the femoral vein, mottling of the limb, and absent distal pulses. Without treatment, phlegmasia cerulea dolens can progress to cause venous gangrene and ultimate limb loss. In the reported case, phlegmasia cerulea dolens developed in a middle-aged woman with metastatic thyroid carcinoma following Greenfield filter placement via the femoral vein after heparinization for deep-vein thrombophlebitis had failed.