Legionellosis is a lung infection caused by bacteria of the genus Legionella. This disease has been monitored as a notifiable disease since 1987. Between 1998 and 2008, out of a total of 11,147 cases of Legionnaires' disease recorded nationwide, 1,983 clinical strains were identified by the National Legionella Reference Center (i.e., 18%). Of these, 1,844 were strains of Legionella pneumophila serogroup 1 (Lp1). To date, three predominant endemic strains of Lp1 have been observed in mainland France: the "Paris," "Lorraine," and "Louisa" strains. Among the other endemic strains, since 1998, approximately 40 exhibited the "Mondial" profile (representing about 2%). The objective of this study was to describe the individual characteristics of patients infected with the "Mondial" Lp1 strain. This initial study demonstrated a relationship between corticosteroid use and infection with the "Mondial" strain, as well as a geographic distribution of this strain exclusively associated with the Île-de-France region. It is necessary to continue monitoring the ecological distribution of "Mondial" strains to confirm whether or not they are adapted to an environment exclusive to the Île-de-France region. In the coming years, the mechanisms of interaction between the environment and the bacterium in terms of adaptability and spread must also be investigated to better understand the virulence of this strain. (R.A.)