The usual way to decide why hyponatremia or hypernatremia has developed and to plan goals for its therapy is to analyze events in electrolyte-free water (EFW) terms. We shall demonstrate that an EFW balance does not supply this information. Rather, one must calculate mass balances for water and sodium plus potassium separately (a tonicity balance) to understand the basis for the change in natremia and the proper goals for its therapy. These points are illustrated with a clinical example.