There have been important recent advances in the diagnosis of acute viral infections of the nervous system. Polymerase chain reaction amplification of nucleic acids in cerebrospinal fluid and tissues is a rapid and accurate tool in the diagnostic evaluation of patients with suspected infections. It has proved to be highly valuable in the noninvasive diagnosis of herpes simplex virus encephalitis and also in establishing the role of herpes simplex virus infection in Mollaret's meningitis. Improved diagnosis of herpesvirus infections should lead to more appropriate antiviral therapy and better outcomes.