The phylogenetic trees of H3N2 HA and NA showed seasonal clusters but also co-circulating lineages within seasons. The introduction of the A/Fujian/411/02(H3N2) strains in 2002-2003 caused a "jump" in the evolution of both HA and NA genes. Many of the substitutions in HA introduced with the A/Fujian/411/02(H3N2)-like viruses have become fixed, probably reflecting a very fit virus. The genetic variation before the 2003-2004 season may have been more influenced by introduction of new viruses through viral migration than adaptive evolution of the genes. A constant rate of drift was not observed for HA but instead periods of change followed by stasis periods. The low dN/dS ratios (0.232 for HA and 0.247 for NA) also indicated that the influenza genes were not directly influenced by positive selection. Reassortments between co-circulating strains and viruses from previous seasons, and introduction of viruses from other parts of the world might play a larger role than natural selection for some seasons, as also observed by others [27, 28] . [2, 4, 25, 26] . The reassorted H1N2 viruses possessed only the HA from A/New Caldonia/20/ 99(H1N1)-like viruses and the rest of the genome from A/ Moscow/10/99(H3N2)-like viruses as also reported by others using partial sequences [25] . The H1N2 viruses have been introduced to Denmark from elsewhere and are not a local reassortant. (Table 1) suggesting they may be important for viral escape from the host immune system and the overall fit of the virus.