Lumbar cerebrospinal fluid (CSF) pressure was recorded in 116 adult neurosurgical patients in the lateral and sitting positions. The level of zero CSF pressure while in the sitting position (ZPS) and hydrostatic indifferent point (HIP) for lateral and sitting positions were determined and referred to the craniospinal axis. In control patients ZPS was located mainly at the upper cervical region, and showed nearly the same variation and frequency distribution as CSF pressure in the lateral position when efforts were made to reduce sources of error and there was no orthostatic change in CSF filling pressure. Under these circumstances ZPS may be used as a variable comparable from one subject to another. In control patients the HIP was located between C-6 and T-5. In 25 hydrocephalic patients, shunting resulted in a mean caudal shift of ZPS of 244 mm, and a mean pressure fall of 126 mm H2O in the lateral position. This difference was due to a caudal shift of HIP on shunting. A caudally located ZPS was found in patients with complete cervical subarachnoid block. Prevention and treatment of CSF leakage cranial to HIP is discussed.