Essential thrombocythemia (ET) is a chronic myeloproliferative disorder characterized by thrombo-hemorrhagic complications. But aortic thrombus formation is not so common in the patients with ET at the initial diagnosis. We describe a 65-year-old woman with ET found to have a thoraco-abdominal aortic thrombosis and have splenic infarction, which were successfully treated with medical therapy alone. To our knowledge, only two cases were published with presenting large aortic thrombosis at the onset.