A 21-year-old man with isolated adrenocorticotropic hormone (ACTH) deficiency complained of loss of consciousness in association with hypoglycemia. Both plasma ACTH and cortisol levels were low and failed to respond to corticotropin-releasing hormone (CRH) stimulation. The patient also showed abnormal findings in hematological examination, such as neutropenia and anemia with lymphocytosis, activity of coagulation factors, and electroencephalography (EEG). Furthermore, mitogen-induced lymphocyte proliferation was increased. After successful replacement therapy with hydrocortisone 15 mg/day, most of these abnormalities including the lymphocyte proliferation were fully restored.