Legionellosis is transmitted through the inhalation of water aerosols contaminated with various species of Legionella. These bacteria are often found in man-made water systems, and their growth is accelerated in hot water. Hot water distribution systems and cooling towers in air conditioning systems are the most commonly identified sources of infection. In France, surveillance of Legionnaires' disease began in 1987 with a mandatory reporting system for clinicians. Between 1988 and 1995, an average of fifty-four cases were reported each year, and six clusters of nosocomial cases were identified. To determine the level of underreporting and assess the feasibility of a laboratory-based reporting system, a survey was conducted in the laboratories of public hospitals. The cases identified through this survey were matched with cases identified in the reporting system and at the National Legionella Reference Center to estimate the number of cases diagnosed in France in 1995. (adapted from the text)