The efficacy of cerebrospinal fluid (CSF) shunting surgery for normal pressure hydrocephalus (NPH) is difficult to predict. The CSF removal test is useful but quantification of the results is difficult. A method to quantitatively measure cerebral blood flow (CBF) by single photon emission computed tomography twice within 30 min after double injection of N-isopropyl-p-[(123)I] iodoamphetamine using a background subtraction method to correct for the temporal profile was utilized in tandem with CSF removal via a lumbar spinal tube in 22 patients of NPH to produce maps of baseline CBF and quantitative CBF change after CSF removal. All 22 patients with NPH underwent ventriculoperitoneal shunting surgery and were divided into two groups according to improvement in clinical symptoms and signs (responder group, N=15; nonresponder group, N=7). Baseline clinical characteristics and baseline CBF values were not significantly different between the two groups. Regional and whole brain CBF changes in the responder group (range 98-105%, whole brain 101+/-39%) were significantly higher than those in the nonresponder group (range 41-48%, whole brain 46+/-40%) (P<0.01). Discrimination analysis showed that an increase of more than 80% in CBF after CSF removal was predictive of response to shunt surgery with 77% accuracy. This new quantitative CSF removal test could be useful for selecting good candidates for CSF shunting surgery among patients with NPH.