A 76-year-old man was first referred to his local hospital at the beginning of August 2005 with nausea and vomiting, and was admitted on August 15 with progressive fatigue of unknown etiology. Gastrointestinal examination was performed, but no obvious abnormalities were detected in the upper or lower digestive tract. Hematology tests also revealed no abnormalities, except for slight eosinophilia. He developed depression and was given an antidepressant agent. After suffering from hyponatremia and disorientation, he was subsequently admitted to our hospital on August 28. At that time, serum adrenocorticotropic hormone (ACTH) and cortisol levels were low, while both the ACTH and corticotrophin-releasing hormone (CRH) stress tests showed no response. Other stress tests revealed normal responses, so he was given a diagnosis isolated ACTH deficiency and received corticosteroid therapy. In summary, we report an elderly case of isolated ACTH deficiency with nonspecific initial manifestations such as nausea, fatigue, and depression.