Regression analysis and analysis of significance was performed using SPSS 11.5. The variance was expressed as squared correlation coefficients (R 2 ), interpreted as follows: < 0.1 very weak correlation; 0.1-0.25 weak; 0.25-0.50 moderate; 0.5-0.75 strong; 0.75-0.9 very strong; > 0.9 exceptionally strong correlation. Table 1 presents the clinical data in 17 countries during eight seasons. When using the norm of +/-1 week, the mean total overlap of sentinel clinical and virological data was 72% (median 71%; range 25%-100%) for countries using ILI as case definition, and 71% (median 73%; range 57%-83%) for countries using ARI as case definition. When using the norm of +/-2 weeks, the mean overlap for ILI-countries rose to 84% (median 86%; range 50%-100%) and rose to 85% (median 86%; range 71%-100%) for countries using ARI as case definition. Table 1 also compares the sentinel clinical data with the non-sentinel virological data. When using a match of +/-1 week, the mean overlap of sentinel clinical and non-sentinel virological data for countries using ILI was 63% (median 67%; range 0%-100%), and 46% (median 50%; range 43%-50%) for countries using ARI. When accepting a match of +/-2 weeks, the overlap rose to 84% for ILI (median 86%; range 50%-100%), and to 68% for ARI (median 60%; range 50%-100%). The differences between ILI and ARI were statistically not significant (1week match, p = 0.26; 2-week match, p = 0.19). Table 2 No correlation was found between length of the epidemic and type of virus or virus combinations. Table 2 assesses the west-east and south-north spread of influenza activity in Europe after respectively plotting the longitude and latitude of each country to the peak clinical level of activity for each season. As an example the plot of the 2003-2004 season is shown in Figure 1 . The assessment was based on calculation of the squared correlation coefficient. In four consecutive seasons (2001) (2002) (2003) (2004) (2005) , there was a moderate to strong correlation between longitude and peak influenza activity, indicating a west-east spread of influenza. It is noteworthy that during these four seasons A(H3) was the dominant virus. However, this was also the case during the 1999-2000 and 2006-2007 seasons, in which there was no indication for a west-east spread. We repeated the analysis for the latitudes of each country and found a moderate correlation in three season, favouring a south-north spread. During two of these seasons (2001-2002 and 2004-2005) there was also a clear west-east spread of influenza. Taking all eight seasons into account the overall indication for a west-east spread was twice as strong than for a south-north spread (mean R 2 west-east: 0.277; mean R 2 south-north: 0.137).