Chronic subdural hematoma (CSH) is a disease caused by minor head trauma and can be cured by surgical treatment. It remains unclear why CSH can cause neurologic dysfunction such as hemiparesis, although some reports describe the contribution of impaired cerebral blood flow (CBF) and oxygen metabolism of patients with CSH. We studied five hemiparetic patients with unilateral CSH using positron emission tomography (PET). Before the operation, both the regional CBF (rCBF) and regional cerebral metabolic rate of oxygen (rCMRO2) were observed to have decreased slightly in the motor cortex and the lentiform nucleus on the bilateral side. However, regional oxygen extraction fraction (rOEF), regional cerebral blood volume (rCBV) and cerebral vascular response (CVR) in the same regions were normal. In the thalamus, no significant findings were observed. Although rCBF and rCMRO2 recovered to within normal range in the motor cortex and lentiform nucleus on the bilateral side by four weeks after the the operation, hemiparesis improved in all the patients within 3 days after the operation. In conclusion, this study suggests that a circulatory disturbance in the motor cortex under the hematoma was not indicated as a definite cause that induced hemiparesis with CSH.