In late May 2009, a surveillance system for clusters of influenza cases was established to assess the circulation of the A(H1N1)2009 virus in the population. On Monday, June 22, the principal of the Jeu de Paume elementary school in Créteil (Val-de-Marne) reported high absenteeism due to flu-like symptoms, and following confirmation of cases of influenza A(H1N1)2009, the SAMU set up a tent clinic in the schoolyard on June 24 and 25. On June 26, the InVS Unit for the Île-de-France region and the Val-de-Marne Departmental Directorate of Health and Social Affairs began an investigation to describe the event and verify the source of the cluster of cases. In total, 68 people (40 students, 2 teachers, and 26 parents or relatives) presented with confirmed or probable influenza. Only 6 people, including 4 elementary school students, underwent testing for the virus. Three students tested positive for the A(H1N1)2009 virus via PCR; the fourth tested positive for influenza A but negative for H1. Two children who developed symptoms on June 18 were likely the index cases. None had traveled or been in contact with anyone who had traveled abroad. In the absence of the onset date of symptoms for the cases, the dynamics of the influenza outbreak were determined using the absenteeism rate. This rate, which was around 3% prior to the event, had reached 11% on the Friday preceding the report and 21% and 18%, respectively, on Monday, June 22, and Tuesday, June 23, before the school’s permanent closure. These clusters of cases involving a large number of people helped confirm the circulation of the influenza A(H1N1)2009 virus in France and led to changes in the methods for monitoring and managing new cases. (R.A.)