BACKGROUND In many nosocomial Legionella outbreaks water distribution systems are the most frequent source of infection. OBJECTIVE Considering the hospital waterline old age, an investigation on colonization by Legionella spp was carried out in order to evaluate the pipeline system weaknesses and to implement environmental preventive measures. METHODS From 2004 to 2010, overall 97 samples from the water line were collected. The samples were analyzed according to the italian Legionella spp standard methods; water temperature, pH and residual free chlorine were determined at the time of collection. X2 test, exact-test and t-test were used to compare proportions and means. RESULTS Overall 28 samples (23.7%) were positive for Legionella spp, and five of them (17.9%) exceeded the threshold level >104 cfu/L. The number of positive samples varied along the years, showing a significant increasing trend (X2 for trend = 11.5; p<0.01), but most occurred in 2008 (53,6%), when the hospital underwent major building reconstruction. Samples positive for Legionella spp by comparison to negative ones showed a lower free chlorine concentration (0.08 mg/L vs 0.15 mg/L) and a higher water temperature (46.1° vs 42.7°). Actually the percentage of positive samples decreased significantly with the increasing in free chlorine in the water (X2 for trend = 8.53; p<0.01). The samples collected at the connection between public water line with the hospital supply network were always free from Legionella. All hospital buildings were colonized by Legionella spp, although 80% of samples >104 cfu/L occurred in the C-building. No cases of nosocomial legionellosis were reported during the study period. CONCLUSIONS Hospital water system showed a diffuse colonization by Legionella spp, although the degree of contamination reached the threshold level (>104 cfu/L) only in a small percentage of samples, showing a substantial effectiveness of the control measures applied.