Background: Legionnaires' disease is a type of severe pneumonia caused by Legionella bacteria. The mortality rate in this disease is 5-10%. People with various comorbidities, smokers and the elderly are at greater risk of developing the disease. Objective: The aim of the work is to present the results of an epidemiological investigation into the outbreak of Legionnaires' disease that occurred in the city of Rzeszów and the surrounding area in August and September 2023 and to present the threat related to the presence of Legionella bacteria in water supply installations and networks. Material and methods: The material for this publication was data from an epidemiological investigation conducted in the outbreak of Legionnaires disease in Rzeszów in 2023. Results: Epidemiological investigation revealed 165 cases of Legionnaires' disease in the outbreak, including 152 confirmed cases and 13 probable cases. The mortality rate in a legionellosis outbreak was 15%. In the course of the investigation, environmental tests were carried out in residential and public buildings and industrial installations. As part of environmental tests, 187 water samples were collected, including 87 samples of warm water. Conclusions: The outbreak of Legionnaires' disease in the city of Rzeszów draws attention to the potential threat from the Legionella bacteria to the health and life of especially elderly people suffering from chronic diseases. The environmental tests carried out confirmed the highest number of Legionella bacteria at medium and high levels in water samples taken in the private apartments of sick people. Despite the lack of strict legal regulations clearly specifying the obligations regarding periodic disinfection of internal hot water supply installations, cooperation with their owners should be undertaken to enforce plans and actions in this area.