Systemic dehydration and diffuse central nervous system (CNS) signs without other illness can be referred to as dehydration encephalopathy (DE). However, clinical features of DE experiencing neurologic emergency have not been fully delineated. In the Emergency Unit and Neurology Department at our university hospital, we studied our patients with DE. Definition of DE: patients who had acute diffuse CNS signs on arrival, which ameliorated after 500-1000 ml saline infusion on admission day, and systemic and blood test findings suggestive of dehydration. Diffuse CNS diseases other than DE such as encephalitis, hypoxia, and drug abuse, and focal CNS diseases such as stroke were excluded. As a result, our patients with DE were uniformly elderly (mean age, 80 years), and all had an underlying brain disease, e.g., ischemic white matter lesions (WML), Alzheimer`s disease, etc. In addition, many showed frontal hypoperfusion in SPECT and high-amplitude slow waves in EEG. Provoking events in many included mild systemic infection, diuretics use, etc. They typically showed acute presentation of diffuse CNS signs (consciousness disturbance in most, new onset of epilepsy in some) and achieved complete recovery quickly (mean period 4 days). Some showed multiple silent spots in diffusion weighted MRI. In conclusion, it is likely that elderly persons with WML, frontal hypoperfusion, and electroencephalographic high-amplitude slow waves are prone to develop DE. Silent spots in diffusion weighted MRI indicate alteration in coagulation or in the blood-brain barrier function. Appropriate management successfully reversed DE. DE is an emerging clinical entity in the elderly that neurologists may encounter. Elderly brain is susceptible to dehydration, which recently attracted attention, both clinically and experimentally. Though mild, initial management is extremely important. In order to avoid unnecessary medication, differentiation of DE from stroke should also be performed.