Hyponatremia, albeit common in chronic renal insufficiency, necessitates a detailed search of the underlying hidden causes. We report on a 67-year-old woman with chronic kidney disease (creatinine 230 μmol/L) and hypertension who suffered from general fatigue, dizziness, nausea, vomiting and abdominal fullness off and on for 6 months. Hyponatremia (plasma Na+ 106-125 mmol/L) on 4 occasions during the past 6 months was noticed. Her extracellular volume status was apparently normal. Plasma Na+ concentration 110 mmol/L was the most striking laboratory abnormality with mild metabolic acidosis (HCO3- 19.8 mmol/L). Her urine Na+ concentration and osmolality were inappropriately high. Her hyponatremia was refractory to normal saline, hypertonic NaHCO3 and 0.1-μg 9α-fludrocortisone. Despite normal plasma cortisol and thyroid hormone concentrations, a provocation test with cosyntropin (250 μg) showed a blunted cortisol (<579 nmol/L) but intact aldosterone response. Magnetic resonance imaging of her brain displayed a normal pituitary gland and hypothalamus. A history of intermittent intravenous steroid therapy to treat her allergic rhinitis for 3 years was uncovered. Steroid supplements induced water diuresis and corrected hyponatremia to 135 mmol/L in 5 days. With nonspecific clinical symptoms, glucocorticoid insufficiency must be kept in mind as a cause of hyponatremia even in patients with impaired renal function and normal plasma cortisol concentration.