Healthcare-associated Legionnaires' disease often leads to fatal respiratory tract infection among hospitalized patients. In this report, three cases of Legionnaires' disease among patients in two different hospitals (Hospital A and Hospital B) were investigated. After conducting an epidemiologic and environmental investigation, the water distribution systems (WDSs) were identified as the possible source of infection, as Legionella pneumophila serogroup 1 (Lp1) was isolated from both clinical and environmental samples. Patients received aerosol therapy with nebulizers during their hospitalization. Based on the results of the investigation, the hospitals' infection control committees reviewed their policies for Legionnaires' disease prevention and implemented control measures focusing on using sterile fluids for aerosol treatments.