From June 29 to July 1, 1998, four cases of Legionnaires' disease in British nationals were reported to the National Public Health Network through the mandatory reporting system and the European surveillance network for travel-associated Legionnaires' disease. One of the cases had been detected through enhanced surveillance during the World Cup. All of these individuals had stayed in Paris within the 10 days prior to the onset of symptoms. An investigation was conducted to confirm the existence of an outbreak, identify a common source of contamination, and implement control measures appropriate to the epidemiological situation. The results of the epidemiological, microbiological, and environmental investigations indicate that the spread of Legionella-contaminated aerosols from a cooling tower appears to be the cause of this outbreak.