Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a rare disease characterized by hyponatremia and hyperosmolality of urine. There are well known causes of this syndrome, such as neoplasmatic processes, different disorder of central nervous system, lung diseases and side effects various drugs. In differential diagnosis of this syndrome we ought to consider renal failure, adrenocortical deficiency (Addison's disease), hypothyroidism, different forms of hyponatremia as well as pseudohyponatremia.