BACKGROUND: Herpes simplex virus encephalitis (HSVE) is a severe disease resulting in high mortality rates and residual sequelae; therefore, early diagnosis and treatment are very important. The purpose of this study was to evaluate the usefulness of polymerase chain reaction (PCR) assay of HSV DNA in cerebrospinal fluid (CSF) from patients with encephalitis for confirmatory diagnosis of herpes simplex virus encephalitis. MATERIALS AND METHODS: PCR analysis was done to detect DNA of HSV in 31 CSF samples obtained from 21 patients with encephalitis, admitted between November 1994 and June 1996, at the Seoul National University Children's Hospital. As controls, 15 samples from patients with proven diseases other than encephalitis were tested. Two different 22 bp- rimers, complementary to sequences within the DNA polymerase regions of HSV gene, were used. After amplification, PCR products were digested with DdeI to confirm the sequence. RESULTS: Three CSF specimens from 2 out of 21 patients with encephalitis were positive for HSV DNA, whereas all the control specimens were negative. PCR was positive in CSF samples obtained at 3 and 7 days after onset of symptoms from one patient and at 5 days from the other. PCR analysis yielded negative results in CSF samples collected at 10 and 14 days after acyclovir treatment. CONCLUSION: In this study, HSVE was diagnosed by PCR amplification of HSV genome from CSF obtained at early stage of the disease from patients with encephalitis. PCR assay of CSF samples could be used for early and confirmatory diagnosis of HSVE.