The etiology, pathophysiology, clinical features, diagnosis, and medical treatment of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) are reviewed. SIADH is a common cause of hyponatremia in hospitalized patients. Increased concentrations of antidiuretic hormone (ADH) result in retention of free water, increased excretion of sodium, and hyponatremia. Symptoms generally occur only when hyponatremia is severe (less than or equal to 125 meq/L) and may include anorexia, vomiting, and confusion, followed by seizures, coma, and death. SIADH may result from a variety of diseases, as well as from the use of drugs such as chlorpropamide, carbamazepine, diuretics, and some antineoplastic agents. Diagnosis of SIADH is confirmed by demonstration of a high urine osmolality with a low plasma osmolality, in the absence of diuretic use. Immediate treatment of the symptomatic patient with SIADH includes intravenous furosemide and 3% sodium chloride injection to produce a negative free-water balance. If the underlying cause of SIADH cannot be corrected, the treatment of choice for chronic SIADH is fluid restriction. If this is not tolerated by the patient, demeclocycline can be used to induce a negative free-water balance. Urea, lithium, phenytoin, and loop diuretics have been reported to be effective, but there are few data to support their use. Future research into the treatment of SIADH must be directed at developing effective antagonists of ADH. Treatment of SIADH consists of elimination of underlying causes and restriction of fluid intake; if these measures are unsuccessful or poorly tolerated, long-term drug therapy may be indicated.