The analysis in this paper is largely based on clinical sentinel reports and it was therefore important to validate this S: Sentinel, information provided by sentinel general practitioners NS: Non-sentinel, data obtained from non-sentinel physicians, hospitals and institutions ILI: influenza-like illness used as case definition by 13 countries ARI: acute respiratory disease used as case definition by 4 countries 1 week overlap: occurrence of peak incidences differ 1 week or less (good match) 2 week overlap: occurrence of peak incidences differ 2 weeks or less (moderate match) Percentages represent the mean overall match aggregated from data of most participating countries obtained during 8 influenza seasons data source, first with the virological sentinel data and then with non-sentinel laboratory data. We found that the data were valid, irrespective of whether ILI or ARI was used as case definition. There was an overall match of 72% (+/-1 week) between the clinical and virological sentinel data and a 60% match (+/-1 week) between sentinel clinical data and the non-sentinel virological data. Allowing a larger overlap (+/-2 weeks) provided a match of 84% and 80%, respectively. The results indicate that the sentinel system is very adequate in estimating influenza activity in a continent, irrespective of case definition. The strength of the system is that it combines communitybased clinical and virological data and also can provide age-specific data. The sentinel approach has already been implemented in other continents, our results imply that it should be considered in other parts of the world [14] .