A case of essential thrombocythemia (ET) associated with subdural hematoma and postoperative intracerebral hemorrhage was reported. A 57-year-old man had complained headache in the morning. Six hours later he was found unconsciousness and soon he was brought to our hospital. On admission he was comatose. There was no evidences of head injury and the X-rays were normal. A computed tomography (CT) scan revealed an acute subdural hematoma over the left temporoparietal region. Laboratory data revealed thrombocytosis of 85.7 x 10(4)/mm3 with increased red and white blood cell counts. Emergent right craniotomy was performed and a subdural clot was evacuated. Neither cortical damage nor vascular malformations were seen on the cortical surface. But a spurting cortical artery with a pin-hole could be seen. A postoperative CT scan revealed an intracerebral hemorrhage deep in the right hemisphere. Cerebral angiograms revealed no vascular anomalies. Postoperatively, the platelet count remained high and laboratory data including bone marrow finding, neutrophil alkali-phosphatase score and chromosome analysis were consistent with the diagnosis of essential thrombocythemia. The mechanisms of subdural hematoma formation and postoperative intracerebral hemorrhage associated with essential thrombocythemia were discussed.