OBJECTIVE: To assess the molecular epidemiology and risk factors for Pseudomonas aeruginosa colonization and infection in hospitalized patients. METHODS: In a 1000-bed university hospital, newly admitted patients were assessed prospectively for colonization and infection with P. aeruginosa. Anal swabs were obtained upon admission and at discharge. Ribotyping was used for the typing of isolates. Epidemiologic and clinical data were recorded prospectively. Independent risk factors were assessed using multivariate analysis. RESULTS: The recovery rate of patients with P. aeruginosa from anal specimens on admission was 6.7% (42/628). Infection due to P. aeruginosa was observed in 20 of 628 (3.2%) patients, of whom 10 (1.6%) were already infected on admission. Independent risk factors for colonization/infection on admission were age, indwelling urinary catheter, the presence of wound and seropositivity for HIV. Independent risk factors for nosocomial infection were anal colonization on admission, alcoholism, indwelling urinary catheter and antibiotic treatment during hospitalization. Ribotyping revealed that 27 patients were colonized or infected with a unique ribotype, whereas 24 shared one or more ribotypes with other patients. Analysis of epidemiologic and molecular typing data revealed that transmission from patients to patients or from patients to environment was documented on only three occasions. CONCLUSIONS: A great many P. aeruginosa strains were isolated from patients and the environment, and most environmental strains were different from those recovered in patients, suggesting a low rate of hospital acquisition of P. aeruginosa from the environment. The main risk factors for hospital-acquired infection were detectable colonization on admission, antibiotic treatment and urinary catheter.