Introduction - On August 19, 2009, several cases of gastrointestinal symptoms accompanied by fever that occurred at a vacation center in Haute-Savoie were reported to the Departmental Directorate of Health and Social Affairs (DDASS). Nasopharyngeal swabs confirmed the presence of the A(H1N1)2009 virus. In accordance with national protocol, an investigation into the cluster of influenza A(H1N1)2009 cases was initiated by DDASS 74 and the InVS unit in the Rhône-Alpes region on August 21. Methods - This was a retrospective study. It included 26 children from the Paris region (12 girls and 14 boys; median age: 8 years) and 6 adults (camp counselors) who stayed at the camp from August 17 to 30, 2009. Cases were investigated using a standard questionnaire (demographic data, date of symptom onset, clinical signs and progression over time, mode and conditions of transportation to the vacation center). Results - Twenty-one (21) children met the definition of a confirmed or probable case (incidence rate = 80%). Among adults, 50% met the definition of a probable case. The epidemic peak was reached on August 19 at 5:00 a.m. The last cases occurred on August 20. All patients had traveled on the TGV. The average incubation period was estimated at 41 hours (range: 3–71 hours). Among the 24 people who presented with flu-like symptoms (21 children and 3 adults), respiratory symptoms were reported by 83% of cases, headaches occurring on the first day by 80% of cases, and vomiting by 48% of patients. No severe cases were reported. The child considered the index case was ill when the train departed. Most of the children had no close contact with this child. Conclusion - This outbreak allowed for the rapid identification of a cluster of influenza A(H1N1) cases at a vacation center and enabled early intervention. The dynamics of the outbreak directed the investigation toward identifying a specific mode of transmission. The very sudden onset of this outbreak (the night of Wednesday, August 19) suggested common exposure to an index case. The strong hypothesis of transmission during the TGV journey raises questions about the role played by the train’s ventilation system. Clinical symptoms described as very common (headaches, gastrointestinal symptoms) and not particularly suggestive of a flu-like syndrome were reported. Finally, this episode highlighted the importance of implementing early containment measures to prevent the spread of the outbreak throughout the center. (R.A.)