The hospitalized patient is at risk of hyponatraemia caused by reduced electrolyte free water clearance and prescription of hypotonic fluids. Hospital-acquired hyponatraemia is common and associated with increased mortality/morbidity. Hyponatraemia in itself can cause severe cerebral symptoms. Small decreases in P-[Na+] in patients with reduced intracranial compliance (e.g. meningitis) can be dangerous. To reduce iatrogenic hyponatraemia the understanding of P-[Na+] is fundamental. Next, meticulously prescription of fluid amount/quality and reevaluation as with any other drug is of paramount importance.