A 94-year-old man had with vascular dementia, visual disturbance, hearing difficulty and speech and motor disturbance. He had a history of diabetes mellitus over 50 years. He developed brittle type diabetes. On administration of mixed type insulin (30: 70, 12-18 units in the morning and 6-8 units in the evening), his blood glucose concentrations fluctuated from almost zero to 500-600mg/dl. After change to short acting regular insulin (4-5 units) before each meal and intermediate type insulin (2 units) before sleeping time, extreme hyperglycemia was not observed, but the brittleness with frequent hypoglycemia persisted. The hypoglycemic symptoms were absent at the time of striking hypoglycemia: it was thought that the patient was condition unaware of hypoglycemia. The cause of the brittle diabetes in the extremely elderly was thought to be depletion of endocrine insulin secretion due to marked β-cell reduction and/or β-cell exhaustion secondary to long term duration of diabetes. Daily detailed observation is required to care for such mentally deteriorated patient with brittle diabetes.