We report the outcome of nosocomial legionnaires' disease in three patients who were isolated in the same sterile unit after allogeneic bone marrow transplantation. In all three cases the disease presented with dramatic pulmonary symptoms, and diagnosis was ascertained by direct immunofluorescence on bronchoalveolar fluids. None of the patients underwent seroconversion. This report draws attention to: (1) the fact that bacteriological filters do not ensure absolute security; (2) the need for frequent monitoring of the two factors governing legionella growth, water temperature and chlorination; and (3) the effectiveness of quinolones as a curative and prophylactic treatment of legionnaires' disease in transplanted patients avoiding pharmacological cyclosporin interaction.