The authors report the case of a 50 year old man admitted to hospital with a right hemiplegia and aphasia of sudden onset in whom embolic fragments were found in the left mid and anterior cerebral artery territories at left carotid angiography : transoesophageal echocardiography demonstrated a protrusive plaque of atherosclerosis in the ascending aorta and a pediculated thrombosis in the descending aorta. Biological investigations revealed a protein S level of 3% (normal : 70-140%). This case illustrates the acute development of a thromboembolic phenomenon originating from the aortic arch in a patient with a coagulation defect.