An 81-year-old woman suffering from sarcoidosis, chronic renal failure caused by hypertention was treated by valacyclovir 500 mg/day, for the diagnosis of herpes zoster of her right back. Her consciousness gradually became worse, and 3 days after taking the drug, she was sent to the emergency department of the hospital. Her conscious level was E2V2M5 (Glasgow Coma Scale) and myoclonus especially in her lower extremities occurred. Head CT and MRI show no obvious, acute abnormal findings other than chronic ischemic lesions, while an electroencephalogram (EEG) shows periodic synchronous discharges (PSDs) and disorganized background activity. Based on these findings, she was diagnosed as valacyclovir-associated acute encephalopathy. After conservative therapy of maintenance hemodialysis, her consciousness gradually improved, and PSDs disappeared accordingly and background activity of EEG became improved. In this case report, we presented valacyclovir-associated neurotoxicity with PSDs in EEG as potentially a surrogate marker. We should be cautious to use valaciclovir which may cause drug-induced encephalopathy especially in elderly or patients with renal failure even though the dose was adjusted in advance.