A health monitoring system for the "Les Eurockéennes" rock music festival was established in July 2000 by the CIRE Centre-Est, in collaboration with the Belfort DDASS and SAMU 90, and with support from the InVS. This was the 11th edition of an event held annually over three days near Belfort, attracting approximately 25,000 people daily, primarily teenagers and young adults. The surveillance system was designed to rapidly detect any abnormal situation that might require investigation or urgent public health intervention. The report presents the implementation details of this surveillance and its results. In particular, it highlights the feedback provided to all partners mobilized for this event. The experience described here is useful for health services wishing to undertake this type of initiative. (R.A.).