the manuscript. TE arranged appointments with the informants during the course of data gathering and contributed to analyzing the data. Both authors read drafts of the manuscript, made comments and approved the final version. Influenza activity in Europe during eight seasons (1999–2007): an evaluation of the indicators used to measure activity and an assessment of the timing, length and course of peak activity (spread) across Europe BACKGROUND: The European Influenza Surveillance Scheme (EISS) has collected clinical and virological data on influenza since 1996 in an increasing number of countries. The EISS dataset was used to characterise important epidemiological features of influenza activity in Europe during eight winters (1999–2007). The following questions were addressed: 1) are the sentinel clinical reports a good measure of influenza activity? 2) how long is a typical influenza season in Europe? 3) is there a west-east and/or south-north course of peak activity ('spread') of influenza in Europe? METHODS: Influenza activity was measured by collecting data from sentinel general practitioners (GPs) and reports by national reference laboratories. The sentinel reports were first evaluated by comparing them to the laboratory reports and were then used to assess the timing and spread of influenza activity across Europe during eight seasons. RESULTS: We found a good match between the clinical sentinel data and laboratory reports of influenza collected by sentinel physicians (overall match of 72% for +/- 1 week difference). We also found a moderate to good match between the clinical sentinel data and laboratory reports of influenza from non-sentinel sources (overall match of 60% for +/- 1 week). There were no statistically significant differences between countries using ILI (influenza-like illness) or ARI (acute respiratory disease) as case definition. When looking at the peak-weeks of clinical activity, the average length of an influenza season in Europe was 15.6 weeks (median 15 weeks; range 12–19 weeks). Plotting the peak weeks of clinical influenza activity reported by sentinel GPs against the longitude or latitude of each country indicated that there was a west-east spread of peak activity (spread) of influenza across Europe in four winters (2001–2002, 2002–2003, 2003–2004 and 2004–2005) and a south-north spread in three winters (2001–2002, 2004–2005 and 2006–2007). CONCLUSION: We found that: 1) the clinical data reported by sentinel physicians is a valid indicator of influenza activity; 2) the length of influenza activity across the whole of Europe was surprisingly long, ranging from 12–19 weeks; 3) in 4 out of the 8 seasons, there was a west-east spread of influenza, in 3 seasons a south-north spread; not associated with type of dominant virus in those seasons. Influenza has an important impact on societies each season. Surveillance data not only provide valuable information on the burden of disease in the population [1, 2] , but also enables an assessment of whether the vaccine is a good match with the circulating virus [3, 4] . Surveillance may help to plan and allocate health care resources and is important for pandemic preparedness [5] . In addition, the surveillance infrastructure can be used to monitor new emerging respiratory diseases, like SARS or avian influenza in humans [6] . Countries in Europe have shared detailed clinical and virological data via the European Influenza Surveillance Scheme (EISS) since 1996 [7] . This collaborative project is partially funded by the European Commission through the European Centre of Disease Prevention and Control (ECDC) and currently includes 30 countries. The scheme covers a total population of about 450 million inhabitants and an area of roughly 12 million square kilometres. EISS collects two types of data on influenza activity each season: 1) clinical and virological data collected by sentinel GPs and 2) virological data from non-sentinel sources. In the present study the EISS dataset was used to characterise important epidemiological features of influenza activity in Europe during eight winters (1999) (2000) (2001) (2002) (2003) (2004) (2005) (2006) (2007) .