Legionellosis became actual in 60-80s of the 20th century. Legionellosis is caused by Legionella pneumophila and related bacteria. Clinical manifestations of the disease may vary from mild fever (Pontiacs fever) to potentially lethal pneumonia (Legionnaires disease). Main natural habitats of Legionella genus are freshwater and humid soil sources. Nevertheless, nowadays most cases of legionellosis are caused by consumption or use of water from man-made water supply systems, especially when appropriate control measures are not carried out. After entering water supply system, Legionella spp. can be found in all levels of the system - from the water preparation to the consumer. Significant increase of Legionellosis cases (from 0.13 cases per 100 000 residents in 2009 up to 2.20 cases per 100 000 residents in 2011 was observed in Latvia. Our goal was to investigate Legionella spp. contamination patterns in water supply systems.