“Legionnaire's Disease” is an acute infectious disease caused by Legionella of the Legionellaceae family. Clinical forms of legionellosis are diverse, but the risk of severe community-acquired pneumonia (СAP) is the main clinical significance. Unfortunately, as technological progress increases, epidemiological risks of foci of infection associated with artificial water systems (favorable habitat for Legionella) are increasing, which has led to numerous epidemic outbreaks and group cases of legionellosis infection recorded in the world recently. There are no pathognomonic symptoms and signs that allow it to be reliably differentiated from CAP by another etiology for “Legionnaire's disease”. An important role in the early verification of the disease is assigned to taking into account known risk factors of infection (a recent trip accompanied by possible contact with a water spray of a centralized air conditioning system) - age over 40 years, male sex, smoking, chronic concomitant diseases (diabetes mellitus, etc.), immunosuppression and timely laboratory diagnosis. The most accessible and convenient methods of primary diagnostics of community-acquired legionellosis are tests based on the detection of soluble antigen L. pneumophila serogroup-1 in urine. The treatment of Legionellosis CAP is based on antibacterial therapy, levofloxacine and macrolides are the drugs of choice. In this article, a series of clinical observations (2017 - 2019) in one of the multidisciplinary hospitals of Moscow is given as a confirmation of the clinical significance of legionella in the etiology of CAP in hospitalized patients and the need for timely diagnosis of the disease. © 2020 Medical Education. All rights reserved.