The 2004-2005 influenza season in Europe started in late December 2004 and the first influenza activity occurred in the west and southwest (Spain, United Kingdom and Ireland). Influenza activity then moved gradually east across Europe during January and early February 2005, and from late February until late March, most movement was south to north. The intensity of clinical influenza activity in ten out of 23 countries was higher than during the 2003-2004 season, and lower or equal to the 2003-2004 season in the other 13 countries. The highest consultation rates were generally observed among children aged 0-14 years. However, the peak consultation rates due to influenza-like illness or acute respiratory infection were not especially high when compared with historical data. The predominant virus strain was influenza A (83% of total detections) of the H3 subtype (85% of H-subtyped A viruses), with fewer influenza B (17% of total detections) or A(H1) viruses (15 % of H-subtyped A viruses) detected. The vast majority of A(H3) viruses were similar to the reference strains A/Wellington/1/2004 (H3N2) and, subsequently, A/California/7/2004 (H3N2) that are closely related drift variants of the A/Fujian/411/2002 (H3N2) prototype vaccine strain. The B viruses co-circulated with A viruses during the whole influenza season in 11 out of 24 countries. Seven of these were located in the northeast of Europe and in these countries the proportion of B viruses was higher (range: 31-60%) than in the rest of Europe (range: 6-26%). In 13 out of 24 countries the B viruses circulated relatively late in the season. About 43% of all antigenically characterised B viruses were B/Hong Kong/330/2001-like (B/Victoria/2/87 lineage), a strain that is distinguishable from the vaccine influenza B strain, which was a B/Yamagata/16/88 lineage virus. Based on the viruses detected worldwide until February 2005, the World Health Organization modified the composition of the 2005-2006 influenza vaccine from the 2004-2005 season vaccine to include a new A(H3N2) component: an A/California/7/2004 (H3N2)-like virus.