Early detection of AD in its initial or preclinical stage has become increasingly important. We report a 73-year-old woman who was confirmed to have AD by a cortical biopsy performed concomitantly during a shunt operation for hydrocephalus. The triads of hydrocephalus improved after the operation, as did follow-up neuropsychological and FDG-PET findings. However, 2.5 years after the operation, the patient developed cognitive declines with abnormal behaviors, which was considered to be an evolved form of AD from the preclinical stage.