Increased cerebral blood flow (CBF) has been proposed as responsible for the clinical improvement after cerebrospinal fluid (CSF) shunting in patients with normal pressure hydrocephalus (NPH). In order to determine any abnormal CSF-CBF pressure-flow relationships in NPH, measurements of regional cerebral blood flow (rCBF) and regional cerebral blood volume (rCBV) were made before and after lowering CSF pressure (CSFP) in 15 patients with NPH, and in ten patients with presumed hydrocephalus ex vacuo. Maximal reduction of rCBF and rCBV occurred in the territory of the anterior cerebral artery in NPH but no in dementia due to brain atrophy. Both CBF and rCBV increased after lowering the CSFP by lumbar puncture in patients with NPH. Patients with higher preoperative rCBF and maximal increases in rCBR and rCBV after lowering CSFP showed the most consistent clinical improvement after CSF shunting. Evidence is offered that CBF autoregulation is impaired in NPH. The CBF test assists in both diagnosis and selection of patients for CSF shunting.