A 53-year-old male was admitted to our hospital for a high fever. He suffered a change in personality, memory loss and disorientation as well. The findings of cerebrospinal fluid showed monocytosis, but the titers of glucose, C1 and ADA were all normal. Although there was no bacterium in the CSF, the patient's electroencephalography finding was abnormal. We diagnosed his condition as viral meningoencephalitis and started treatment with antiviral agents. Blood chemistry showed serum sodium of 130 mEq/l and plasma osmolarity was reduced to 272 mOsm/kg, while urine osmolarity was high at 353 mOsm/kg. Two potential causes of hyponatremia in this patient were the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) or cerebral salt wasting syndrome (CSWS). Physical findings revealed a contracted extracellular fluid volume, strongly suggesting the presence of CSWS. The massive urine sodium loss overcoming sodium intake supported this diagnosis. After treatment with vigorous sodium and volume replacement for over 4 weeks, hyponatremia as well as meningoencephalitis were improved without any complication. To the best of our knowledge, this is the first report on CSWS in a patient with viral meningoencephalitis.