A mobile thrombus in the carotid arteries is a very rare ultrasonographic finding and is usually diagnosed after a neurological emergency, such as a transient ischemic attack or cerebral infarction. We present the case of a 54-year-old man with vascular risk factors (a heavy smoker, untreated hypertension) who was admitted to the emergency unit with right sided hemiparesis and aphasia. A cerebral CT scan showed a left middle cerebral artery territory infarction. The duplex ultrasound examination revealed mild atherosclerotic changes in the right common and internal carotid arteries, right-sided complete subclavian steal phenomenon and a complicated hypoechoic atherosclerotic plaque in the left common carotid artery with a large mobile thrombus.  Due to the high embolization risk, the patient was hospitalised and prescribed Aspirin together with low molecular weight Heparin. We recorded an improvement in the patient’s neurological status and the control duplex scan revealed disappearance of the thrombus. The presence of floating thrombus in a patient with clinical and imagistic evidence of stroke is a major therapheutic challenge for the neurologist. The treatment strategies are not standardized and must be individualized, however in our case parenteral anticoagulation proved to be successful.