In 1963-1993, several strains of West Nile virus (WNV) were isolated from ticks, birds, and mosquitoes in the southern area of European Russia and western Siberia. In the same regions, anti-WNV antibody was found in 0.4-8% of healthy adult donors. Sporadic human clinical cases were observed in the delta of the Volga River. In spite of this, WNV infection was not considered by the health authorities as a potentially emerging infection, and the large WNV outbreak in southern Russia, started in late July 1999, was not recognized in a timely fashion. First evidence suggesting a WNV etiology of the outbreak was obtained by IgM ELISA on September 9. Two weeks later, the specific WNV RT-PCR was developed and WNV disease was confirmed in all 14 nonsurvivors from whom brain tissue samples were available. Retrospective studies of serum samples by IgM ELISA indicated WNV etiology in 326 of 463 survivors with aseptic meningitis or encephalitis. Moreover, 35 of 56 patients who contracted aseptic meningitis in 1998 had a high titer of WNV IgG antibody, so the WNV infection seems to have been introduced into the Volgograd region before 1999. A complete sequence (AF317203) of WN viral RNA, isolated from the brain of one Volgograd fatality, and partial sequences of an envelope E gene from other nonsurvivors showed that the Volgograd isolate had the greatest homology (99.6%) with WN-Romania-1996 mosquito strain RO97-50.