Hyponatremia is the most frequent electrolyte disorder. Two forms of it, the "true"--and "pseudo"--hyponatremia are known. The normal osmoregulation is an accurate operation which ensures the steadiness of serum sodium level by regulating vasopressin (ADH) release and water intake. Hyponatremia usually indicates water excess in the body, however, it may be complicated by sodium loss as well. It has hypovolemic, hypervolemic and normovolemic forms; the syndrome of inappropriate antidiuretic hormone (SIADH) is associated mostly with the normovolemic states. Nowadays the pathomechanism, criteria, diagnosis and etiologic factors of SIADH (water intoxication) are fairly well known, but the number of drugs capable of inducing this syndrome is increasing day by day. According to the newest knowledge, SIADH may exist not only in the acute but chronic form as well, which should be born in mind when treating water intoxicated patients. The basic principle is that in cases with mild clinical disturbances aggressive treatment should be avoided. For mild hyponatremia water restriction is usually sufficient, but in serious cases hypertonic saline infusion should be administered. Its speed has to be determined and adjusted carefully according to the needs of the patient, and it can be combined with the administration of furosemide, when necessary. Vasopressin antagonists are under clinical investigation, their therapeutic value has not yet been determined. Water intoxication is not rare-if one keeps it in mind. The syndrome's simple treatment can be life saving for the patient and provides an easy problem solution for the physician.