From July 5 to 14, 2008, the 5th edition of the "Armada" boat rally took place in the Rouen metropolitan area in Seine-Maritime (76). During this period, the event welcomed nearly 8 million visitors, including 1.5 million along the Seine during the boat parade down the river to Le Havre. Since any large gathering of people increases health risks, regional health authorities worked together to prepare for the event and to establish dedicated or reinforced emergency response, medical care, and public health surveillance systems during the event. The Interregional Epidemiology Unit (CIRE), in collaboration with the Departmental Directorate of Health and Social Affairs (DDASS) of Seine-Maritime, was tasked with establishing a response system and ensuring epidemiological surveillance during the event. This report presents a summary and assessment of the organization of the gathering and the environmental and epidemiological surveillance measures. (R.A.)