West Nile virus (WNV) is a flavivirus related to tick-borne encephalitis virus endemic in northeast of Poland. WNV circulates among wild birds, transmitted by wide range of haematophagous arthropods. Several mosquito species, especially Culex spp., can transmit WNV to humans. WNV is responsible for a broad range of clinical symptoms, from asymptomatic infection to severe encephalitis and polio-like myelitis. Elderly persons are at especially high risk of death and severe neurological complications, with mortality among patients over 65 years old with WNV-related encephalitis being as high as 35%. Warm and humid climate favors the spread of WNV, however, it can also be effectively transmitted in moderate climate zones. During the last decade a worldwide spread of WNV was observed, with establishment of new enzootic foci and growing number of human cases. WNV seems to be already present in several European countries, including Poland. We describe detection of anti-WNV IgM antibodies in serum of patient with acute febrile infection, which may constitute the first case of confirmed WNV infection in Poland.