The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the commonest form of normovolaemic or dilutional hyponatraemia, and it is caused by either excessive hormone excretion or dysregulated hormone action. Etiology is multifactorial, including drugs, neoplasms, brain and hypophysis lesions, benign lung lesions or human immunodeficiency virus. Clinical features depend on the severity and rate of fall of hyponatremia. Diagnosis demand a normal thyroid, adrenal, liver, renal and cardiovascular function, as well as to rule out diuretic intake. Hypertonic (3%) saline infusion is the treatment of choice of severe acute hyponatremia; whereas fluid restriction is the treatment of asymptomatic chronic hyponatremia. Other pharmacological alternatives are urea, demeclocycline or tolvaptan (the latter having aroused great interest in the last decade).