Hyponatremia is the most common electrolyte abnormality among hospitalized patients. Death or brain damage associated with hyponatremia has been described since 1935, and it is now evident that hyponatremia can lead to death in otherwise healthy individuals. In the past, it had been assumed that the likelihood of brain damage from hyponatremia was directly related to either a rapid decline in plasma sodium or a particularly low level of plasma sodium. Recent studies have demonstrated that other factors may be more important. These factors include the age and gender of the individual, with children and menstruant women the most susceptible. Although many clinical settings are associated with hyponatremia, those most often associated with brain damage are postoperative, polydipsia, pharmacological agents, and heart failure. Morbidity and mortality associated with hyponatremia are primarily a result of brain edema, hypoxemia, and associated hormonal factors. Management of hyponatremia is largely determined by symptomatology. If the patient is asymptomatic, discontinuation of drugs plus water restriction is often sufficient. If the patient is symptomatic, active therapy to increase the plasma sodium with hypertonic NaCl is usually indicated. Although inappropriate therapy of hyponatremia can lead to brain damage, such an occurrence is rare. Thus, the risk of not treating a symptomatic patient for exceeds that of improper therapy.