Herpes simplex encephalitis is the most common cause of sporadic encephalitis in the western world. Patients usually have altered levels of consciousness, altered levels of mentation, fever, headache, and personality changes. These may progress to hemiparesis and seizures. Exact diagnosis must be established by brain biopsy and identification of the virus in biopsy material. There is a great need for a noninvasive test that is positive early in the disease. Without antiviral treatment the mortality rate is greater than 70%, and many survivors have serious disabilities. Both adenine arabinoside and acyclovir decrease death and morbidity, but acyclovir is the preferred drug. With acyclovir about 40% of patients will survive with normal development or minor levels of impairment but more than half of the patients will die or suffer significant impairment. It is essential to treat early; patients who are young and have not reached coma or impaired consciousness may show 65% recovery and return to normal function. Development of new antiviral drugs or other types of therapies is desirable. Herpetic skin lesions are likely to be more confusing than diagnostic because other types of encephalitis with fever often precipitate recurrent herpes that is unrelated to the encephalitis.