We made a diagnosis and therapeutic evaluation of herpes simplex encephalitis using polymerase chain reaction (PCR). Cerebrospinal fluids (CSF) of 24 patients suspected of meningoencephalitis were examined by PCR. HSV (herpes simplex virus) DNA was demonstrated in 3 patients, in which CSF anti-HSV antibody raised, and moreover in another patient, lacking the raise of the HSV antibody. Since this patient was considered to suffer from HSV encephalitis by clinical symptoms and findings on brain CT, this result suggests authenticity of the PCR for detection of HSV infection. HSV-DNA in the 4 patients were semi-quantitated. The amount of the DNA reduced and disappeared corresponding to their improvement of clinical symptoms and laboratory data. Thus, the present method is considered to be useful for rapid diagnosis of HSV encephalitis, and also for evaluation of effects of therapy to decide whether administration of anti-herpes drugs should be stopped.