The results obtained in the laboratory diagnosis of 609 cases of acute or subacute encephalitis, studied during a period of time of even years, is briefly presented. Diagnostic methods included virus isolation from stools and cerebrospinal fluid (CSF); specific serology in serum; detection of intrathecal production of IgG antibody; and, in the last two years, detection of viral genome sequences in CSF by the polymerase chain reaction. Significant results were obtained in 196 cases (32.2%) and the alfa-herpesviruses were responsible for a major part of them (77.5%). Furthermore, 18 cases were likely to respond to dual infection by both herpes simplex and varicella-zoster viruses. Epstein-Barr virus and Human herpesvirus 6 were found in CSF from three immunocompetent patients. Besides the current vaccination program, measles virus is still responsible for an important part (22/196, 11.2%) of cases of viral encephalitis.