Emergence of severe respiratory disease in pet dogs caused by avian-origin influenza A virus, subtype H3N2 (canine influenza virus, CIV), has been reported in Korea since 2007. To ascertain the date of introduction of H3N2 CIV in dog populations in Korea, a serological retrospective study was carried out using archived sera. A total of 980 serum samples gathered between 2004 and 2009 were tested by ELISA, hemagglutination-inhibition assay, and microneutralisation assay. The results indicate that H3N2 CIV has been consistently circulating in dog populations since 2005, two years prior to its first isolation. Influenza A viruses transmit relatively frequently from the avian reservoir to other birds and mammals, yet they do not typically establish permanent lineages in these new hosts (Webster 2002). Nevertheless, such interspecies transmissions have resulted in the establishment of endemic influenza virus lineages in domestic poultry, pigs, horses and human populations (Hansen 2004). Successful interspecies transmission of influenza A virus is dependent on both host and virus genetic factors, and subsequent spread within the new host population requires a period of adaptation of the virus to the new host (Webster and others 1992). In dogs, historically, influenza virus had not been considered to be a pathogen until equine-origin canine influenza subtype H3N8 emerged in the USA in 2004 (Crawford and others 2005). In 2007, the unprecedented transmission of a H3N2 avian influenza virus (AIV) to dogs was reported in South Korea (Song and others 2008). The H3N2 virus was isolated during outbreaks of severe respiratory disease in dogs at multiple facilities. Serological evidence of the infection between June and December 2007 has been detected in …