From June 29 to July 1, 1998, four cases of Legionnaires' disease, including one death, were reported to the National Public Health Network (RNSP) through the mandatory reporting system and the European Legionnaires' disease surveillance network. The patients, British nationals who had traveled to France in June, had in common that they had stayed in Paris within 10 days prior to the onset of symptoms. On July 2, the Directorate General of Health requested that all Departmental Directorates of Health and Social Affairs report daily to the RNSP any cases of Legionnaires' disease reported since June 1. European counterparts were also informed. Following the reporting of new cases among French residents who had visited Paris, a case-control investigation was initiated to identify potential high-risk neighborhoods and exposures. As of the publication date of this article, the results of the investigation suggest that the 20 cases of Legionnaires’ disease that occurred in Paris in June and July may be linked to a common external environmental source of contamination and possibly to the cooling towers of buildings located within a defined perimeter. (adapted from the text)