A 78-year-old woman diagnosed with non-classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency had been under glucocorticoid replacement therapy since the age of 17 years. After several weeks of suffering from gastroenteritis with vomiting, she presented with disturbance of consciousness, hypotension, dehydration, and severe hyponatremia (108 mEq/L) and a markedly increased serum vasopressin concentration (45.5 pg/mL). She regained consciousness after correcting her body-fluid balance with hypertonic saline and intravenous hydrocortisone sodium therapy. Her hyponatremia was likely caused by extra-renal sodium loss and impaired water excretion induced by an increase of serum vasopressin due to volume depletion and glucocorticoid deficiency.