Phlegmasia is a condition that occurs when there is an extensive iliofemoral DVT. Venous and lymphatic outflow are obstructed, and the leg becomes markedly edematous. If untreated, arterial circulation to the lower leg and foot is compromised. Hypovolemia from the third spacing of fluid may cause a decrease in cardiac output, thereby increasing the risk of thrombus extension and pulmonary embolism. About 50% of the cases are associated with a malignancy. Other causes include hypercoagulable states, trauma, and surgery. It can occur at any age but is more common during the fifth and sixth decades of life. The incidence is higher in females than in males.