OBJECTIVE Idiopathic normal pressure hydrocephalus (NPH) is an important cause of dementia in the elderly; however, idiopathic NPH is often difficult to differentiate from normal aging and vascular dementias in which brain atrophy with ventricular dilatation (hydrocephalus ex vacuo or central atrophy) is present. To elucidate the distinctive features of the distribution of CSF in idiopathic NPH, we used MR imaging to investigate the morphologic features and volume of the CSF space in patients with idiopathic NPH compared with those with other dementias. METHODS We assessed the size of four CSF compartments (the ventricle, basal cistern, sylvian space, and suprasylvian subarachnoid space) in 11 shunt-responsive patients with idiopathic NPH by semiquantitative and volumetric analyses of coronal T1-weighted MR images. The results were compared with those in 11 age- and sex-matched patients with Alzheimer disease and in 11 patients with vascular dementia. RESULTS In patients with idiopathic NPH, the CSF volume was significantly increased in the ventricles and decreased in the superior convexity and medial subarachnoid spaces as compared with patients with other dementias. The sylvian CSF volume in patients with idiopathic NPH was significantly greater than in patients with Alzheimer disease. The volume of the basal cistern was comparable among the three groups. In several patients with idiopathic NPH, focally dilated sulci were observed over the convexity or medial surface of the hemisphere. CONCLUSIONS Our results indicate that findings of enlarged basal cisterns and sylvian fissures and of focally dilated sulci support, rather than exclude, the diagnosis of shunt-responsive idiopathic NPH and suggest that this condition is caused by a suprasylvian subarachnoid block.