Background The environmental sources associated with community-acquired or nosocomial legionellosis were not always detectable in the mainland of China and Hong Kong,China.The objective of this study was to illustrate the control measures implemented for nosocomial and community outbreaks of legionellosis,and to understand the environmental distribution of legionella in the water system in Hong Kong,China.Methods We investigated the environmental sources of two cases of legionellosis acquired in the hospital and the community by extensive outbreak investigation and sampling of the potable water system using culture and genetic testing at the respective premises.Results The diagnosis of nosocomial legionellosis was suspected in a patient presenting with nosocomial pneumonia not responsive to multiple beta-lactam antibiotics with subsequent confirmation by Legionella pneumophila serogroup 1 antigenuria.High counts of Legionella pneumophila were detected in the potable water supply of the 70-year-old hospital building.Another patient on continuous ambulatory peritoneal dialysis presenting with acute community-acquired pneumonia and severe diarrhoea was positive for Legionella pneumophila serogroup 1 by polymerase chain reaction (PCR) testing on both sputum and nasopharyngeal aspirate despite negative antigenuria.Paradoxically the source of the second case was traced to the water system of a newly commissioned office building complex.No further cases were detected after shock hyperchlorination with or without superheating of the water systems.Subsequent legionella counts were drastically reduced.Point-of-care infection control by off-boiled or sterile water for mouth care and installation of water filter for showers in the hospital wards for immunocompromised patients was instituted.Territory wide investigation of the community potable water supply showed that 22.1％ of the household water supply was positive at a mean legionella count of 108.56 CFU/ml (range 0.10 to 639.30 CFU/ml).Conclusions Potable water systems are open systems which are inevitably colonized by bacterial biofilms containing Legionella species.High bacterial counts related to human cases may occur with stagnation of flow in both old or newly commissioned buildings.Vigilance against legionellosis is important in healthcare settings with dense population of highly susceptible hosts.