OBJECTIVE: To evaluate the role of faucets as a reservoir for Pseudomonas aeruginosa colonization/infection of patients hospitalized in intensive care units (ICUs). DESIGN: Prospective epidemiological investigation performed during a nonepidemic period of 1 year. The inner part of the ICU faucets were swabbed for P. aeruginosa. Data were recorded on all patients with at least one culture of a clinical specimens positive for P. aeruginosa. Pulsed-field gel electrophoresis was used to characterize the strains. SETTING: Five ICUs of a university hospital which are supplied by two separate water distribution networks. PATIENTS: During a 1-year period 132 cases were investigated. RESULTS: In 42% of cases (56/132) there were isolates identical to those found in the faucets, with a total of nine different genotypes. Among the nine genotypes isolated from both patients and faucets one of them, the most prevalent, was isolated in the two networks and in 30 cases. The other eight genotypes were recovered almost exclusively from either one (three genotypes in 12 cases) or the other (five genotypes in 12 cases) network and from the patients in the corresponding ICUs. CONCLUSIONS: These results suggest that the water system of the ICUs was the primary reservoir of patient's colonization/infection with P. aeruginosa in a substantial proportion of patients, although the exact mode of acquisition could not be determined.