Serum osmolality is one of the end-points in the management of neurologic intensive care patient. Its leading role in the concept of cerebrovascular homeostasis is underlined. Normal plasma osmolality is generally 285 mOsm kg-1, a value determined almost entirely by small molecules and ions (Na+, K+, urea and lactate). The plasma osmolality value is determined by measuring the changes in freezing point related to the zero value of a sample of distilled water. The measurement of plasma osmolality is very easy and inexpensive; its widely use could be very useful in the neurologic intensive care units to improve the treatment of neurological critical patient. According to the authors the monitoring of plasma osmolality should be mandatory to evaluate the effectiveness of treatment of brain edema.