Since it was first recognized 35 years ago, the syndrome of inappropriate antidiuretic hormone (SIADH) secretion has become the most commonly recognized cause of hyponatremia among hospitalized patients. The syndrome is caused by excessive intake of fluids when urinary dilution is impaired by physiologically inappropriate secretion or administration of vasopressin or other antidiuretic hormones. Inappropriate secretion of the hormone may be ectopic by a malignancy or ectopic and can ensure from any of three different types of osmoregulatory defects. Rarely, there is no demonstrable defect in the osmoregulation of vasopressin. The excessive fluid intake may be due to inappropriate thirst but often is iatrogenic. The syndrome occurs in association with many diseases, particularly of the lungs and brain, and can also be caused by drugs or surgery. Its principal symptoms are neurologic and due to the associated hypo-osmolality of body fluids. Diagnosis requires exclusion of certain other hormonal or hemodynamic disorders that can also increase vasopressin or impair urinary dilution. Therapy differs depending on the severity and duration of the hyponatremia but is always based on cautiously raising plasma sodium by fluid restriction, infusion of hypertonic saline, or administration of drugs that block the antidiuretic effect of vasopressin.