We described 10 cases of idiopathic normal pressure hydrocephalus (NPH) in elderly patients who had good results after shunting. Retrospective analysis of our cases offers the following results which indicate the pathogenesis and surgical selection of idiopathic NPH. 1) Most of the patients had risk factors of ischemic cerebrovascular disease and had multiple small ischemic lesions such as lacunae. 2) Cerebrospinal fluid (CSF) pressure is below 20mmHg in all cases and its response to acetazolamide loading was suppressed or almost lost in most of cases. 3) The volume of the lateral ventricle was slightly reduced after the effective operation. 4) Some of the patients showed clinical improvement even if they had symptoms for more than 2 years before treatment. Two primary theories exist concerning the cause of NPH. One is extraventricular obstruction to CSF flow at the level of the arachnoid villi or the basal cistern. The other is periventricular white matter ischemic damage which decreases the tensile strength of the ventricular walls and leads to ventricular dilatation. Tangential periventricular shearing force due to ventricular dilatation produces the symptom of NPH. Our results support these descriptions as the causes of idiopathic NPH in elderly patients. We noticed, firstly, involvement of the ischemic risk factor. Other findings implied decreased elasticity or compliance of the brain and the loss of the autoregulatory properties of cerebral circulation which accelerate the ischemic damage to the periventricular white matter. In understanding the etiology, it is important to know the fragility of the aged brain and the mechanical and hemodynamic stress to which it is subjected.(ABSTRACT TRUNCATED AT 250 WORDS)