In 1997, the incidence of Legionnaires' disease in Europe was 3.9 cases per million inhabitants. The lowest incidence rates were reported in Malta (0) and Norway (0.2), and the highest in Denmark (24). Italy reported only 90 cases (1.5 cases per million inhabitants), 20% of which were nosocomial, but undiagnosed legionellosis and underreporting appear to be the main reasons for this low incidence. In May 1998, an isolated case of nosocomial Legionnaires' disease was discovered in Milan at a 1,000-bed hospital, in wards housing immunocompromised patients or those with serious conditions (transplant recipients, AIDS patients, patients on corticosteroids, etc.). This outbreak provided an opportunity to conduct a long-term epidemiological and environmental investigation. Thus, after 15 months of active surveillance, recommendations were made regarding measures to be taken in response to Legionnaires’ disease acquired in hospital or community settings. (R.A.)