A 35-year-old woman presented with a 2-day history of progressive left lower extremity pain and swelling. She had started oral contraceptives 4 months prior, and had flu-like symptoms that required her to stay home for the past week. Physical examination confirmed marked swelling extending from the left ankle to the groin. An erythrocyanotic appearance existed throughout the involved limb. Duplex ultrasonography revealed acute thrombosis within the calf, popliteal, femoral, and common femoral veins. The iliac vein was technically difficult to insonate. Due to suspicion of a coexistent iliac deep venous thrombosis (DVT), a left lower extremity venogram was obtained that confirmed acute thrombosis within the left iliac and common femoral veins (Figure 55-1). This presentation and clinical constellation of signs and symptoms was consistent with May-Thurner syndrome (MTS).