['To assess the diagnostic potential of the polymerase chain reaction (PCR) in herpes simplex virus (HSV) encephalitis.', 'Samples of CSF from 516 patients with encephalitis were studied for HSV-DNA by PCR.', 'Samples taken one to 29 days from the onset of symptoms from 38 patients (7.4%) were positive, 32 (6.2%) for HSV-1 and six (1.2%) for HSV-2. At follow up, eight of 28 patients studied were still HSV-PCR positive. A diagnostic serum:CSF antibody ratio to HSV but not to other viruses was detected in 25 of the 38 HSV-PCR positive patients thus supporting the initial PCR findings. Patients positive by HSV-PCR were concentrated in the age group > or = 40 years, and especially in patients aged 60-64 years, of whom nine of 24 (37.5%) were positive. The HSV-PCR was negative in all other patients with encephalitis of known or unknown aetiology. This group included 34 patients with a diagnostic serum:CSF antibody ratio to other viruses. A dual infection, HSV and another microbe, was considered possible in seven patients.'] Is herpes encephalitis a disease of middle aged and elderly people : polymerase chain reaction for detection of herpes simplex virus in the CSF of 516 patients with encephalitis . The Study Group?