A 62-year-old man was admitted because of nausea and vomiting. Severe hyponatremia with renal sodium loss was found. Endocrinological studies revealed that the patient had isolated adrenocorticotropin (ACTH) deficiency and secondary adrenocortical insufficiency. Furthermore, an inappropriate secretion of antidiuretic hormone (ADH) in relation to the low plasma osmolality was observed at an early stage of hyponatremia. Hydrocortisone therapy effectively corrected his hyponatremia. Following the correction of hyponatremia, the value of free water clearance increased and the level of the plasma ADH decreased. Thus, the present case indicates that ACTH deficiency can cause the syndrome of inappropriate secretion of ADH.