The relationship of clinical manifestations to hematoma volume and regional cerebral blood flow (r-CBF) was analyzed in 65 adult patients with chronic subdural hematoma (CSH). Hematoma volume was measured in computed tomographic scan images with a semi-automatic analyzer (Leitz ASM), and r-CBF by ¹³³Xe inhalation method. Headache, hemiparesis, and later consciousness impairment occurred with increase of hematoma volume. Headaches due to increased intracranial pressure were common in younger patients, while a third of the aged cases with neurological deficits complained of no headaches. Older patients who retained CSH over 3 months showed mental symptoms. Neurological deficits occurred in cases with over 95 ml of hematoma in the 40 to 60-year age group, and over 120 ml with those aged over 60 years. The degree of the midline shift in unilateral CSH was not always correlated with hematoma volume. Such differences in the critical hematoma volume and the degree of midline shift were probably due to age-dependent changes in the brain weight and intracranial free space. Twenty cases with unilateral CSH showed bilateral decrease in r-CBF. The r-CBF in the hemiparesis group decreased especially at the frontal and parietal lobes, ipsilateral to the hematoma.