Hyponatremia is an electrolyte disorder frequently encountered in clinical practice. The first step in the diagnostic evaluation of hyponatremia is the volume assessment (clinical estimation of hypo/hypervolemia presence). In the cases of euvolemic hyponatremia, followed by decreased plasma osmolality (<275mOsm/kg) with urine osmolality >100mOsm/kg, urine sodium >30mmol/L along with normal intake of salt and water, preserved pituitary, adrenal, thyroid, adrenal function as well as no recent use of diuretics, syndrome of inappropriate diuresis (SIAD) is diagnosed. In symptomatic, especially acute hyponatremia, careful and gradual substitution by 3 (10) % sodium-chloride infusion is advised with liquid intake restriction, the administration of demeclocycline, urea, or vaptans, as well as causal treatment of SIAD. SIAD is often transient and etiologically clear. In this paper, we present two cases with idiopathic, chronic SIAD with poor outcomes. During the diagnostic follow-up, the cause of SIAD was not found. The autopsy finding was also adverse. In most cases, euvolemic hyponatremia in patients with chronic, idiopathic SIAD is an ominous sign.