In order to investigate the pathogenesis of normal pressure hydrocephalus (NPH), dementia was analyzed with regard to the characteristic preoperative picture, the time when dementia improved, and what aspect of dementia was improved, retrospectively. Twenty seven patients of NPH were divided into two groups; young adult group: patients under 64 years old and geriatric group: patients above 65 years old. We found that the only difference found between these two groups was severity in dementia grading, higher in geriatric group. Postoperative course or complications did not differ. Dementia in NPH was not an essential impairment of intellectual ability, but was characterized by the subset symptoms of dementia, such as decrease in spontaneity and interest, slowness and paucity of thought and action. Improvement of memory followed postoperative recovery of these subset symptoms which was observed at earliest three days after shunt surgery. We postulate that recovery of dementia in NPH is associated with not only a decrease in intracranial pressure but also an improvement in the microcirculation of the brain or an improved metabolism.