Legionella pneumophila is a common cause of nosocomial and community – acquired pneumonia. The primary ecological niche of Legionella appears to be a parasite of protozoa, but humans may become secondarily infected after inhaling or aspirating organisms. Legionella are found in freshwater environments or wet soil, but major reservoirs are manmade aquatic environments, particularly hot water systems. There were several reports about Legionella cases in Riga (Latvia); however, the source of L.pneumophila has not yet been proven. Therefore, the aim of study was to investigate the occurrence of Legionella pneumophila in water supply systems of apartment buildings in Riga and to evaluate the correlation between presence of Legionella pneumophila in drinking water distribution system and prevalence of Legionella antibodies in blood samples of healthy residents of Riga.