We encountered a patient with siolated ACTH deficiency accompanying dementia. A 69-year old man was admitted because of abdominal pain. A decrease of serum Na level (114mEq/l) due to increased urinary Na excretion suggested the existence of adrenal insufficiency. While serum cortisol and urinary 17-OHCS, 17-KS excretions were reduced, plasma ACTH level remained below the normal range. Since daily ACTH injections increased both serum cortisol level and urinary 17-OHCS excretion, the patient was diagnosed as having isolated ACTH deficiency. The severity of dementia was measured by using the Hasegawa Dementia Scale. Both cerebral blood flow and metabolism, assessed with Positron Emission Computed Tomography (PET), decreased even after the start of 20mg/day cortril administration. An increase of the cortril supplement dose to 30mg/day attenuated the reduction of cerebral blood flow with the improvement of dementia from score point "1" to "27" (Hasegawa scale). The present case raised the possibility that loss of glucocorticoid may involve the development of dementia, resulting from decreased cerebral blood flow and metabolism.