The tall ship gathering known as "Les Voiles de la liberté" was first held in 1989 to mark the bicentennial of the Revolution. The event, which took place in the commercial port of Rouen (Seine-Maritime), concluded with a parade down the Seine. Health risks may be heightened during large gatherings of people, particularly due to: ¿ the concentration of people at the same time and in the same place, which can facilitate the spread of diseases with epidemic potential; ¿ diseases imported from countries where these conditions are endemic; ¿ the establishment of temporary food service operations, which may pose greater risks regarding adherence to the cold chain and sanitary conditions for food preparation despite the control measures in place; ¿ weather conditions that could lead to heatstroke; ¿ fights or brawls; ¿ accidental injuries. A simple epidemiological surveillance system was implemented during the event. The objectives of this surveillance were: ¿ to prevent and rapidly detect any abnormal health event defined as any condition requiring immediate preventive, prophylactic, or health control intervention by health authorities, or any situation of an epidemic nature; ¿ to assess health risks that could lead to epidemic diseases; ¿ to assist health authorities in decision-making regarding intervention in the event of a potential epidemic. The daily analysis of the data collected in the morning (corresponding to information from the previous day or from earlier in the day if necessary) was intended to detect any health event in the population in order to trigger early public health interventions aimed at controlling the spread of diseases with epidemic potential, namely: ¿ investigation to identify a common source of contamination (as is the case, for example, with a mass foodborne illness outbreak); ¿ prophylaxis for contacts of a disease such as invasive meningococcal infection; ¿ investigation of a bioterrorism alert; ¿ dissemination of prevention messages: limiting the effects of extreme heat. The Cire, in collaboration with the Ddass of Seine-Maritime and the InVS, was tasked with implementing the surveillance system and ensuring epidemiological surveillance during the event. The purpose of this report is to summarize the organization of the gathering, present the environmental and epidemiological surveillance system, and assess its effectiveness. Several points for discussion will then be presented for readers’ consideration.