Regional cerebral blood flow and regional cerebral circulation time were measured in 13 demented patients with chronic hydrocephalus, mostly normal pressure hydrocephalus. The average hemispheric, frontal, and temporal cerebral blood flows were significantly reduced. The average regional cerebral circulation time values were significantly prolonged in the frontal, temporal, and thalamic regions, most markedly in the frontal white matter, where periventricular lucency was observed on computed tomography. Clinical improvement was obtained in all patients after operation. While postoperative regional cerebral blood flow values did not change compared with preoperative ones, postoperative regional cerebral circulation time values were significantly reduced in all the regions measured, and most markedly in the frontal white matter. The present results suggest that microcirculation in the frontal lobe is closely correlated with dementia in association with pressure exerted on the nerve fibers in the frontal white matter in patients with normal pressure hydrocephalus.