Idiopathic normal pressure hydrocephalus (iNPH) is a condition with dilated cerebral ventricles but intracranial pressure within normal limits. The symptoms of gait impairment, cognitive decline and urinary incontinence develop gradually. Treatment with shunt insertion results in improvement in eight out of ten patients. The cerebrospinal fluid tap test (CSF TT) and preoperative magnetic resonance imaging (MRI) are methods used to select patients who may benefit from shunt surgery, but they are performed and interpreted differently in different centers throughout the world. The aim of this thesis was to evaluate the performance of the CSF TT and the underlying mechanisms of improvement in gait function after CSF removal, and to investigate the prognostic value of preoperative MRI scans. Improvement in gait and changes in cerebral blood flow (CBF) after a CSF TT were investigated in two prospective studies that included 39 and 20 patients, respectively. Gait assessment and perfusion MRI were done before and several times during the first 24 hours after a CSF TT. Perfusion was investigated with pseudo-continuous arterial spin labeling. At the group level, gait function was significantly improved at all investigation times, but only one-third of individual CSF TT responders were improved at all investigation times. In patients with increased CBF in lateral and frontal white matter after the CSF TT, gait function improved more than it did in patients with decreased CBF in these regions. However, in the whole sample, there was no significant increase in CBF after CSF removal. Preoperative MRI scans were retrospectively evaluated in 109 patients with iNPH who had undergone shunt surgery. The callosal angle was smaller in shunt responders compared with non-responders. The following findings showed the highest association with a positive outcome after shunting: a small callosal angle, wide temporal horns, and occurrence of disproportionally enlarged subarachnoid space hydrocephalus. In conclusion, CBF in white matter close to the lateral ventricles may play a role in the reversibility of symptoms after CSF removal in patients with iNPH. The CSF TT should be reevaluated if the patient does not initially improve, and preoperative MRI investigations can add prognostic information regarding the selection of shunt candidates.