Hyponatremia and hypernatremia are among the most common electrolyte disorders. Since the plasma sodium level is determined by the ratio between the total quantity of effective solutes (primarily sodium and potassium salts) and the total body water, abnormalities in the plasma sodium level must be produced by a change in one or more of these parameters. In most patients, alterations in body water are of primary importance because the plasma sodium level is normally regulated by changing water intake and water excretion. Measurement of free water excretion has traditionally been calculated by using a formula that includes the urine osmolality. However, urea is a major urinary solute but does not contribute to regulation of the plasma sodium level, since it is an ineffective osmole. As a result, urinary solute excretion is best expressed as 2 X UNa+K. Making this important correction allows solute and water intake and excretion to be compared, thereby leading to a better understanding of both the development and correction of disturbances in the plasma sodium level.