The effect of inspiration and expiration on the systolic slope of the cerebrospinal fluid (CSF) pulse wave has been studied in 83 shunted and non-shunted patients undergoing diagnostic tests for suspected hydrocephalus. A ratio of the systolic CSF pulse slope on inspiration to the same in expiration (I/E ratio or index) has proved statistically valid in identifying non-hydrocephalic patients from hydrocephalic patients and in separating hydrocephalic patients into arrested, communicating and aqueductal stenosis hydrocephalus. The I/E ratio depends on the comparative damping effect of intracranial venous venting on the systolic CSF pulse slope during inspiration (I) when venous volume is evacuated from the cranium by negative mediastinal pressure, and during expiration (E) when cranial venous volume flow to heart is minimal due to positive mediastinal pressure. The low cranial venous outflow on expiration produces little effect on the normal damping of the systolic CSF pulse slope. The high venous outflow on inspiration produces a loss of damping, causing a high systolic CSF pulse slope. Therefore, exhausted cranial venous volume, or exhausted intracranial compliance, produces an I/E ratio approaching 1.0, whereas a normal I/E ratio is between 2.0 and 3.0. The I/E ratio can presumably be used to assess intracranial compliance changes occurring before the dangerous late intracranial pressure (ICP) upward surge related to the volume-pressure curve in all clinical problems of increasing ICP. The I/E ratio may be used likewise to assess the urgency of treatment for any hydrocephalus and increased intracranial pressure problem, i.e. the closer to unity the greater the urgency.