A 41-year-old woman was admitted to the emergency room with symptoms compatible with deep vein thrombosis affecting the left lower extremity and pulmonary thromboembolism. A CT scan was consistent with pulmonary emboli and thrombosis of the iliac veins extending to the inferior vena cava, which persisted even after correct systemic fibrinolytic therapy. For this reason, a venography was performed and local thrombolysis was administered. Venography revealed a compression of the left common iliac vein caused by the right common iliac artery, so that the patient was diagnosed with May-Thurner syndrome. The clinical features of this anatomical condition and sometimes lethal clinical syndrome are discussed.