The first hospital-acquired case of legionnaires' disease in Poland was detected in the ophthalmology ward of a large hospital. Analysis of medical files revealed previous suspected cases of legionnaires' disease in the ophthalmology ward. Biological samples confirmed legionnaires’ disease. The source of the bacteria proved to be the old hot water system. Chemical and heat-shock treatment of the pipelines diminished the bacterial load but further technical interventions were necessary to eliminate all bacteria. The engineers removed dead legs from the pipelines, as well as the old large metal hot water tanks. The hospital installed an automatic chemical disinfection system. Legionella disappeared and no new cases of the disease were observed.