Objective: We have confirmed by PCR-amplification ofthe DNA polymerase gene, three cases of Herpes Sim-plex encephalitis (HSE) with atypical clinical symp-toms.Methods: Cerebrospinal fluid (CSF) samples were col-lected by lumbar puncture during the course of test-ing from 23 patients suspected to have viral encepha-litis based on commonly encountered symptoms, cy-tochemical analysis and neurological testing. PCR wasdone on CSF using specific primers that amplify a 290base pair sequence on the DNA polymerase gene com-mon to both HSV-1 and HSV-2 strains.Results:  Only 3 of the 23 CSF samples collectedshowed the expected amplicon size on the target se-quence. One case had conventional symptoms of en-cephalitis and 2 cases showed atypical symptoms fromthe conventional ones. All 3 patients recovered afteradministration of acyclovir. Fourteen PCR-negativepatients were shown to have non-HSE viral encephali-tis and the remaining 6 patients were shown later tohave other neurological diseases.Conclusion:   PCR was efficient in elucidating the atypi-cal cases of HSV encephalitis.Key words: HSV, encephalitis, PCR