The possibility of West Nile virus (WNV) activity in Japan is discussed in reference to WNV ecology and epidemics in the USA, where the virus appeared in 1999 and rapidly expanded its distribution area with increasing numbers of patients. The arrival of WNV in Japan would occur through three transporting systems: WNV-infected mosquitoes carried mostly by plane; import of infected pet birds; and migration of infected wild birds. Once in Japan, the spread and symptoms of WNV disease among human beings and horses would be more serious and drastic in Japan than in the USA. The reasoning for this is that the level of cross-immunity of Japanese against WNV induced by the smaller wave of recent epidemics of the closely related Japanese encephalitis virus must be lower than that of Americans due to the larger wave of recent epidemics of the closely related Saint Louis encephalitis virus. Public participation as well as governmental efforts would be a vital necessity to prevent the arrival of WNV and to control epidemics after arrival. As countermeasures, surveys of three subjects should be conducted first: vector mosquitoes inside planes from WNV epidemic regions, including the USA; dead birds infected with WNV; and vector mosquito breeding sites. Second, reduction of breeding sites and/or application of insect growth regulator (or insecticides) should be carried out in and around dwellings and workplaces. Third, citizens should be encouraged to protect themselves from mosquito bites.