The sixth edition of the Armada de Rouen, a major international gathering of sailboats and military vessels, took place from June 6 to 16, 2013. Given the nature of the event, the Cire Normandie stepped up its public health surveillance measures for the occasion. Routine public health surveillance, conducted using tools linked to networks of healthcare professionals, was supplemented by a public health surveillance system specific to the event. This system relied on the daily collection and quantitative and qualitative analysis of data from several local healthcare providers. As a result, the health surveillance dedicated to the event made it possible to collect and classify 1,478 reports in which the need for medical care was related to the event. The majority were treated by the on-site first-aid teams. An evaluation of the system demonstrated the effectiveness of health data collection and analysis at both the event site and the metropolitan area levels. However, computerizing the coding and transmission of data from all stakeholders, combined with situational coding, would have allowed for a more rapid response in analysis and management in the event of a potential unusual, slow-onset public health incident. Furthermore, the system revealed its limitations in the epidemiological monitoring of potential rapid-onset public health events, which implies the need to plan ahead by developing a specific system in advance of such events. (R.A.)