On February 27, 2003, one of the participants in a European scientific meeting held on February 14 and 15, 2003, in Lyon reported to the event organizer that four participants, including himself, had developed acute gastroenteritis following the meeting. One of them was hospitalized for Guillain-Barré syndrome. The clinical signs, particularly Guillain-Barré syndrome, suggested a Campylobacter spp. infection. This information was forwarded to the coordinator of the European Enter-net network, who notified the French Institute for Public Health Surveillance (InVS). Thirty-two people had attended the meeting. Non-French participants were staying at the same hotel. Four meals provided by the meeting organizers were served. An epidemiological investigation was conducted by the InVS in collaboration with the Departmental Directorate of Health and Social Affairs (DDASS) and the Departmental Directorate of Veterinary Services (DDVS) of Rhône-Alpes to confirm the epidemic nature of the outbreak, identify the causative pathogen, determine the source of the outbreak, and propose appropriate control and prevention measures. (Introduction)