Since it is unknown how cerebrospinal fluid (CSF) pressure within the craniospinal space is regulated, in this paper we examined the influence of body position and CSF volume on CSF pressure in the cranial and spinal space in 19 patients with normal pressure hydrocephalus. Using two volumetric techniques for cranial (CIVET and volBrain) and semi-automatic segmentation for spinal space, it was observed that the cranial CSF volume is about 300 ml and the spinal CSF volume is 114 ml, which is a much larger volume of CSF than was previously thought. Withdrawal of 15 ml of CSF volume by lumbar puncture leads to a decrease in CSF pressure by average 8.5 cm H2O, which indicates that the CSF volume in the spinal space has a great influence on the level of CSF pressure. The measurement of CSF pressure in the horizontal position in the cranial and lumbar subarachnoid spaces indicates that the CSF pressure in the entire craniospinal space at the same hydrostatic level is same, while lifting the body by 15° decreases the pressure in the cranial space and increases the pressure in the spinal space. The observed difference in pressure between cranial and spinal space corresponds exactly to the hydrostatic difference. For the first time, it was observed that patients who had a positive clinical response to prolonged external lumbar drainage had a significantly higher CSF spinal volume (around 120 ml on average) indicating a new predictive factor in the treatment of patients with normal pressure hydrocephalus. The results indicate that CSF pressure behaves like hydrostatic pressure and does not depend on the secretion, circulation and absorption of cerebrospinal fluid, which is consistent