The aim of the study was risk assessment for exposure to legionellae measured by seroreactivity of in-patients and infection in colonized hospitals. Sampling of water from distribution systems of two hospitals (A, B) revealed legionella contamina- tion in 30.7 % and 28.3 % in the ranges of 104-107 CFU/1. The most common isolates were of Legionella pneumophila sp. (L.p.) serotypes L.p.10 and 5 in hospital A, in hospital B L.p.l (non-Pontiac group) and L.p. 3 (some of tested strains were virulent for guinea pigs). Paired sera of 64 patients from hospital A in the ages of 56 ± 16.0 taken in the range of 18.4 ± 8.6 days reacted mostly with low titers (up to 1:32) with standard antigens (15 patients) and endemic antigens (6 patients). Sera (time spán 25.0 ± 8.1 days) of 64 patients with oncological diseases (from hospital B) aged 56.2 * 7.3 years reacted only with standard antigens in lower titres in 4 cases. Despite of low seroreactivity of in-patients and no nosocomial cases in colonized hospitals, both facilities (especially hospital B) must be considered as risk sites for susceptible patients, active surveillance using urinary legionella antigen detection must be introduced.

        Key words: colonization of water by legionellae - seroreactivity of patients