We reported a 85-year-old woman with cerebral thrombosis associated to essential thrombocythemia (ET) under antithrombotic therapy. She had past histories of bronchial asthma and a pacemaker implantation due to sick sinus syndrome. She had had antiplatelet and anticoagulant therapy during the past several years after coronary artery bypass surgery and paroxysmal atrial fibrillation. She developed the cerebral infarction in left medial pons, presenting disturbance of consciousness, dysarthria, and right hemiparesis. The carotid ultrasonography showed the severe stenosis or occlusion in the distal vertebral-basilar artery in Doppler flow pattern, furthermore, the thrombotic occlusion of the right internal carotid artery with low-moderate echoic lesion. The increase in platelet counts (129×104/μl) was diagnosed as ET. After the additional cytoreductive therapy started, Doppler flow pattern of vertebral arteries improved. The pathogenesis of brain infarction was diagnosed as the cerebral thrombosis being associated with the ET in this case. The time course of carotid ultrasonographic findings was useful to understand the clinical pathophysiology of cerebral thrombosis.