Japanese encephalitis virus remains an important cause of encephalitis in Thailand.

Background: Japanese encephalitis virus (JEV) is endemic in Thailand and prevention strategies include vaccination, vector control, and health education.

Methods: Between July 2003 and August 2005, we conducted hospital-based surveillance for encephalitis at seven hospitals in Bangkok and Hat Yai. Serum and cerebrospinal (CSF) specimens were tested for evidence of recent JEV infection by immunoglobulin M (IgM) enzyme-linked immunosorbent assay (ELISA) and a plaque reduction neutralization test (PRNT).