Hypernatremia results when the water content of body fluids is deficient compared with sodium content. Hypernatremia can be the result of pure sodium excess but is usually associated with dehydration, secondary to excess losses of water or hypotonic fluids. Hypernatremic dehydration is less common than hyponatremic or isonatremic dehydration, but is associated with the highest morbidity and mortality rate, primarily related to CNS dysfunction. Except when hypernatremia has developed rapidly, the serum sodium concentration should be corrected slowly with frequent monitoring of serum electrolytes. Even then CNS damage can result, either as a consequence of the hypernatremia itself or of rapid lowering of the serum sodium concentration.