To investigate the causative mechanism of hemiparesis in chronic subdural hematoma, 38 patients with unilateral chronic subdural hematoma were studied on the relationship between their clinical manifestations and regional cerebral blood flow measured with 133xenon inhalation (16 detectors on each side of the head, Initial Slope Index). Twenty-five patients with hemiparesis (hemiparesis group) and 13 patients with headaches only and without any neurological deficits (headache group) were examined before surgery for chronic subdural hematoma. Among the hemiparesis patients, 15 were examined after surgery. Preoperative regional cerebral blood flow values in the headache group were normal in all regions bilaterally and showed no significant regional difference in one hemisphere or interhemispheric difference between the corresponding regions in both hemispheres, whereas preoperative regional cerebral blood flow values in the hemiparesis group were generally around the lower limit of the age-matched normal value and were subnormal in some areas, and the regional cerebral blood flow values were significantly lower on the hematoma side than on the intact side in most regions. The rolandic region especially showed the lowest regional cerebral blood flow value of 32.3 in the Initial Slope Index on average and the most significant interhemispheric differences of regional cerebral blood flow. Such a preoperative reduction of regional cerebral blood flow in the hemiparesis group normalized along with clinical improvement after evacuation of the hematoma. It was suggested that localized cerebral blood flow reduction at the rolandic cortical region under the hematoma might be one of the causative factors of hemiparesis in chronic subdural hematoma.