BACKGROUND Isolated adrenocorticotropic hormone deficiency (IAD) is a rare disorder characterized by central adrenal insufficiency (AI) but normal secretion of pituitary hormones other than adrenocorticotropic hormone. IAD usually presents with unspecific symptoms of AI, such as anorexia and fatigue, but some patients present with a variety of atypical manifestations. Rhabdomyolysis is a potentially life-threatening clinical syndrome caused by skeletal muscle injury with the release of muscle cell contents into the circulation. A wide variety of disorders can cause rhabdomyolysis. Herein, we report an unusual case of IAD presenting with hyponatremia and rhabdomyolysis. CASE REPORT A 67-year-old Japanese woman with a 2-month history of anorexia and fatigue was diagnosed with severe hyponatremia (serum sodium, 118 mEq/L) and rhabdomyolysis (serum creatine phosphokinase, 6968 IU/L), after 2 days of vomiting and muscle weakness. Physical and laboratory findings did not show dehydration or peripheral edema. Her rhabdomyolysis resolved with normalization of serum sodium levels during administration of sodium chloride. However, her anorexia and fatigue remained unresolved. After reducing the amount of sodium chloride administered, the patient still had hyponatremia. Detailed endocrinological examinations indicated IAD; her hyponatremia was associated with inappropriately high plasma arginine vasopressin levels. The patient received corticosteroid replacement therapy, which resolved her anorexia, fatigue, excessive arginine vasopressin, and hyponatremia. CONCLUSIONS This case highlights the importance of considering the possibility of central AI in patients with hyponatremia and excessive arginine vasopressin levels. In addition, rhabdomyolysis associated with hyponatremia can be an important manifestation of IAD.