The genus flavivirus, family Flaviviridae includes a serocomplex of vector-borne neurotropic viruses as in the tick-borne encephalitis virus serocomplex and the Japanese encephalitis virus serocomplex. The tick-borne encephalitis virus serocomplex includes Central European encephalitis virus and Russian spring summer encephalitis virus. West Nile virus (WNV), which in the Western hemisphere was first detected in New York, in the United States in 1999, is one of several mosquito-borne members of the Japanese encephalitis serocomplex, that cause similar encephalitis. The virus has moved to western states of the U.S. since 2002 and spread rapidly to neighboring countries, including Canada, Mexico and Caribbean islands during 2003-2004. Serology has a dominant role in the laboratory diagnosis of WNV and other flaviviruses in humans. This review aims to describe recent developments in the understanding of flavivirus encephalitis to assist in the event of a WNV outbreak in East Asia.