In November 2000, the National Legionella Reference Center (CNR) identified seven strains of Legionella pneumophila serogroup 1 that had been submitted by the bacteriology laboratory at the Rennes University Hospital and that shared an identical genomic profile. These 7 strains had been isolated between August and November 2000, and all 7 patients lived in or had stayed in Rennes during the 10-day period preceding the onset of symptoms. An epidemiological, environmental, and microbiological investigation was then initiated to identify a common source of contamination and propose control measures. (R.A.)