Antiphospholipid syndrome (APS) is characterized by venous and/or arterial thromboses in the presence of the lupus anticoagulant, anticardiolipin antibodies, or both. Antiphospholipid syndrome may occur as a primary disease or in patients with systemic lupus erythematosus or other autoimmune, infectious, or neoplastic disorders. This is a case of APS associated with chronic myelomonocytic leukemia and complicated by a large descending thoracic aorta thrombus responsible for lower limb and visceral emboli. The intraaortic clot was discovered by transesophageal echography (TEE). The association of APS with chronic leukemias has only rarely been described and, as far as can be ascertained, the location of a thrombus in the descending thoracic aorta has never been reported for this syndrome. This case illustrates the crucial role of TEE in the assessment of patients with systemic embolizations.