Introduction - In France, the reporting of clusters of influenza A(H1N1)2009 cases was intended to guide management measures aimed at delaying the spread of the epidemic among the general population. The epidemiological objectives were to detect the onset of local transmission and to monitor the spread of the virus in France. Methods - Between May 14 and September 29, 2009, any cluster defined by at least three cases of acute respiratory infection with sudden onset occurring within one week in the same community was required to be reported to the Departmental Directorate of Health and Social Affairs (DDASS). An investigation was conducted, and the information was entered into a database. Outbreaks were classified after receipt of the results of diagnostic tests. A link to a case returning from abroad was sought. Results - A total of 653 outbreaks were reported, and 253 (39%) tested positive for the A(H1N1)2009 virus. The weekly proportion of clusters with no link to travel abroad increased, rising from 25% in June (week 25) to 80% in September (week 37). The number of departments reporting cases rose from 9 in June to 39 in September. Schools and households accounted for 58% of the clusters. Management measures were implemented and reported for 105 outbreaks (41.3%). Conclusion - Surveillance of clustered cases made it possible to detect the onset of transmission of the A(H1N1)2009 virus in June in France and to track its spread while surveillance of the general population showed no significant increase in influenza activity. From an epidemiological perspective, cluster surveillance proved to be a relevant tool in the absence of other reactive systems for understanding the spread of the virus and describing the affected populations. (R.A.)