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Herpes simplex virus (HSV) is the most common cause of severe viral encephalitis in humans with an approximate annual incidence in the United States of 1 in 250,000. HSV causes progressive cerebral necrosis and edema, with up to 70% mortality if untreated. Of the untreated survivors, about two-thirds have neurological deficits. Effective antiviral drugs are available but are most effective when given early. Diagnosis cannot be based solely on clinical criteria because other diseases may have similar signs and symptoms. Current tests for detection of herpes virus are not sufficiently sensitive or specific for early diagnosis of herpes simplex virus encephalitis. Viral culture of CSF is positive in only about 4% of patients with herpes simplex encephalitis confirmed by brain biopsy. Serologic analysis of simultaneously drawn CSF and serum samples has allowed reliable diagnosis but is rarely positive in the early stages of infection. Early diagnosis of HSV is important to start adequate treatment and to exclude other diseases that have a similar clinical presentation. Because of its excellent sensitivity and specificity in detecting small amounts of DNA, enzymatic amplification of HSV DNA by polymerase chain reaction (PCR) may be a useful diagnostic tool.