A 61-year-old woman with chronic kidney disease presented with general weakness and a confused mentality. Two days before admission she had been diagnosed as Bell's palsy at an outpatient clinic, and started to take oral prednisolone and acyclovir. A blood test at admission revealed hyponatremia (128 mmol/L). After withdrawing acyclovir, her plasma sodium levels began to increase, returning to normal 2 weeks later. This case could provide further evidence for a causal relationship between acyclovir and hyponatremia.