BACKGROUND: Pseudomonas aeruginosa commonly colonizes the hospital environment. Between April 2006 and September 2008, we investigated an outbreak of P. aeruginosa infection occurring in a pediatric intensive care unit. We conducted epidemiologic and molecular investigations to identify the source of the outbreak. METHODS: Retrospective case finding; surveillance cultures of patients and environmental sites; admission screening; case-control study; and molecular typing. PATIENT AND SETTING: Infants and children in a pediatric intensive care unit of a tertiary-care institution. RESULTS: Thirty-seven cases of P. aeruginosa infection or colonization were detected between April 2006 and September 2008, including 3 fatal bloodstream infections. A closely-related strain was detected in 4 residents of a humanitarian nongovernmental organization (NGO) center who developed an infection, from 4 additional residents upon their hospital admission, and from a sink drain at the NGO residential center. NGO recipients represented 65% (24/37) of the total number of cases of P. aeruginosa colonization or infection during the outbreak period. Investigation at the residential center showed widespread contamination of the sewage system (10/14 sinks and shower drains, 70%) and a high prevalence (38%) of P. aeruginosa carriage among children. CONCLUSIONS: These findings suggest that the probable cause of the outbreak was the contamination of the NGO residential center with further nosocomial transmission after admission, and highlight the importance of considering external sources when investigating hospital outbreaks.