Posthypoxic and postencephalitic myoclonus is often poorly controlled with current treatments. The authors successfully treated three patients with posthypoxic and postencephalitic myoclonus by using levetiracetam, a new antiepileptic drug. Levetiracetam appears to be a promising agent for treating action myoclonus caused by hypoxic and encephalitic brain injury-the degree of functional improvement may depend on the severity of associated motor dysfunction.