Herpes simplex encephalitis (HSE) is one of many types of viral encephalitis. Epidemiologic estimates of incidence range from 1000 to 2000 cases per year in the United States. This accounts for only 5% to 10% of the 20 000 reported cases of encephalitis that occur yearly. The usual clinical presentation of encephalitis is an acute febrile illness with altered behavior, decreased level of consciousness, and focal neurologic deficits. Herpes simplex encephalitis is distinguished from other causal agents by being the most common cause of sporadic fatal encephalitis in the United States with a 70% mortality rate in untreated patients. Clinically, HSE commonly shows early focal neurologic deficits on physical examination and by computed tomography, electroencephalogram, or brain scan.1 Unfortunately, many other viral illnesses, bacterial infections, and any acute mass lesion can present with a focal basal temporal localization that mimics HSE.1-3 Studies using vidarabine and, subsequently, acyclovir