Quantitative electroencephalography from the occipito-parietal region, gait, and psychometric tests were performed in patients with idiopathic normal pressure hydrocephalus (NPH) before and after drainage of 40 ml CSF. The results were compared to those in demented controls. The EEG results were also analyzed with respect to possible correlations with clinical, CSF dynamics, and psychometric variables. Significant differences between NPH patients and demented controls were not found neither in baseline EEG variables nor regarding the CSF tap-test-related change in EEG. The EEG-response to the CSF tap-test did not often exceed the day-to-day test-retest variability. In NPH patients, EEG slowing (relative delta and theta power) correlated significantly with CSF outflow-resistance (p < 0.002). Relative theta power correlated with the Thurstone's Identical Forms test (p < 0.05). Significant correlations between EEG and Bingley's Memory Test were found for the control group but not among NPH patients (p < 0.05). Thus, EEG band-power variables seem to reflect some of the pathophysiology involved in idiopathic normal pressure hydrocephalus, particularly increased resistance to CSF outflow. The practical value of the presently applied EEG method in NPH-diagnosis and prognostic evaluation seems to be limited, however.