Adrenocortical insufficiency such as occurs in Addison's disease causes hyponatremia and renal tubular acidosis (RTA). Hyponatremia results from both aldosterone and cortisol insufficiency. RTA is due to aldosterone insufficiency. The involvement of cortisol in RTA is unclear. Here, we report a woman in her 70s who was admitted to our hospital with severe hyponatremia (106 mEq/l) and RTA. The patient exhibited low plasma cortisol levels with little response to rapid adrenocorticotropic hormone loading. In contrast, the plasma aldosterone concentration was maintained at or above the normal range. Hydrocortisone replacement greatly improved both the hyponatremia and RTA. This case suggests that both aldosterone and cortisol are involved in acid secretion from the kidney.