A 24-year-old man with disseminated herpes zoster, which occurred 9 months after bone marrow transplantation for chronic myeloid leukemia, developed encephalopathy and immobilizing myoclonus after 7 days of vidarabine treatment (10 mg/kg of body weight per day). Only mild hepatic dysfunction was a risk factor for a toxic reaction. After the vidarabine therapy was stopped, the symptoms worsened until treatment with hydration, large doses of chlordiazepoxide, and protective care gave symptomatic relief.