The case of a female patient with a suprasellar optic glioma is reported, who was admitted to the intensive care unit due to decompensated diabetes insipidus with hypernatremia of 194 mmol/l. The sodium concentration was reduced slowly over 4 days and the patient recovered without sequelae. Based on this case the article deals with the pathophysiology and therapy of hypernatremia. An increase in extracellular osmolarity leads to augmented production of intracellular osmolytes in order to maintain the cell volume constant. Due to this counterregulation correction of the sodium concentration must be done with caution.