Herpes simplex virus (HSV) meningoencephalitis has a high mortality rate if proper antiviral therapy is not applied. Thus rapid diagnosis is of peculiar importance in such cases. In this study we aimed to evaluate the use of polymerase chain reaction (PCR) and detection of intrathecally synthesized antibodies by serological methods in viral meningoencephalitis suspected cases to determine HSV as the causative agent. Seventeen cases with cerebrospinal fluid (CSF) samples with microscopical and biochemical findings compatible with viral encephalitis were included in this study. CSF samples yielded no bacterial growth. Cell cultures propagated in Vero cell line and PCR with different primer sets against HSV-1 and HSV-2 (specific for US7 and US2 gene regions, respectively) were used to investigate the presence of HSV in CSF. Serum samples were taken simultaneously with the CSF sampling and "Reibergram" graphics and antibody index (Al) calculations were used for the evaluation of intrathecal antibody synthesis. Albumin and total IgG levels in serum and CSF samples measured with nephelometry (Dade-Behring, Germany) for Reibergram graphics, albumin and IgG ratios were calculated. Quantitative levels of HSV 1+2 IgG were measured in serum and CSF samples using ELISA (HSV-1 12 Pool; Antibody Determination in CSF, Euroimmun, Germany) for Al determination. One of the CSF samples obtained one week after the neurological symptoms had started, yielded HSV-2 in cell culture and also HSV-2 DNA was detected by PCR. Intrathecal antibody synthesis was not detected in this case. In two cases with symptoms lasting for more than three weeks, intrathecal IgG synthesis in Reibergrams and pathological intrathecal HSV Al (27.9 and 7.9, respectively) were detected, however, virus isolation and PCR detection were not successful. For the other 14 cases, HSV-DNA were found negative and no intrathecal antibody synthesis were detected. HSV meningoencephalitis can be diagnosed via using PCR which has been accepted as gold standard in recent years, with 24 hours turn-around time. However, if CSF samples were taken later than the first week following the beginning of symptoms, possibility of HSV detection by PCR is lowered. According to the data obtained from this limited study, it may be suggested that PCR may be used for the detection of HSV-DNA in CSF samples during the early phase of meningoencephalitis cases, however, consideration must be taken to detect the intrathecal antibodies during the later phases of the infection where intrathecal antibody synthesis starts. Key words: Meningoencephalitis, herpes simplex virus, cerebrospinal fluid, polymerase chain reaction, antibody index, Reibergram.