Hyponatremia is associated with important morbidity, that includes the often fatal central pontine myelinolysis. It occurs more frequently in patients treated with diuretics, in liver cirrhosis, congestive heart failure and in the elderly. A sodium (Na) imbalance should be considered as highly probable in the presence of delirium, confusion, inappropriate behavior and coma. In the majority of cases hyponatremia is caused by Na depletion. This is associated with volume depletion, which, in turn, triggers thirst. The consequent reintroduction of solvent without solutes reconstitutes the volume lost, whilst further diluting Na. Less frequently an excess of solute-free water introduced orally or infused intravenously is retained in the presence of a reduced renal diluting capacity. Hyponatremia due to water excess may be distinguished from that caused by solute depletion by careful history taking, physical examination and by measurements of the body weight. Simple formulas, easily applicable to the bedside allow an accurate estimate of the water excess or solute deficit, and hence an accurate and effective quantitative correction of the alteration.