CNS infections caused by human herpesvirus 1 and human herpesvirus 2-herpes simplex virus type 1 and herpes simplex virus type 2--are reviewed. The major diseases associated with these viruses are meningitis and encephalitis. Two forms of encephalitis are known, neonatal encephalitis and encephalitis occurring after the neonatal period. Both diseases are associated with high mortality and morbidity and require prompt diagnosis and aggressive antiviral chemotherapy. Methods for the specific diagnosis of these infections are reviewed and the value of intrathecal antibody assay and nucleic acid amplification techniques are emphasised.