Hyponatremia is commonly defined as a serum sodium concentration of 135 mEq/L or less, which is reported to occur in 0.97-2.48% of hospitalized patients. Especially, in elderly patients, severe hyponatremia is sporadically developed without any complaints. Although a variety of causes of hyponatremia are documented, diagnosis of functional adrenal insufficiency is difficult because of a lack of the diagnostic criteria. In this case report, we present five cases of hyponatremia in the elderly, who were presumed to have functional adrenal insufficiency, successfully treated with low doses of hydrocortisone.