The pathological changes in the brains of seven patients who had been clinically diagnosed as normal pressure hydrocephalus (NPH) are described and the possible etiological mechanisms are discussed. The pathological findings in all cases consisted of demyelination akin to Binswanger's type of encephalopathy, especially in the frontal lobes. Arteriosclerosis accompanied by occasional organized thrombi and scattered microinfarcts in the periventricular white matter were seen. Focal leptomeningeal fibrosis, diminution of arachnoidal granulations, and non-specific aging processes were noted. Among the above of particular interest, was the degeneration of both periventricular and deep white matters with microinfarcts, and moderate to severe arteriosclerosis. On the basis of these observations, we assume that the degeneration in the white matter is not merely a secondary change due to the result of enlargement of ventricle, but plays an important role in the development of NPH. The development of NPH requires not only the disturbance of cerebrospinal fluid, but also the pre- or coexisting vulnerability in the white matter caused by variables such as ischemia, hypoxia, and trauma.