BACKGROUND: Endoscopes, including bronchoscopes, are the medical devices most frequently associated with outbreaks of nosocomial infections. We investigated an outbreak of Pseudomonas aeruginosa infections after bronchoscopic procedures. METHODS: Microbiologic results were reviewed to determine the rates of recovery ofP. aeruginosa from bronchoalveolar-lavage specimens. Environmental samples from endoscopes and the endoscopy suite were cultured. Medical records were reviewed to identify infections in the 14 days after a bronchoscopy. RESULTS: The rate of recovery of P. aeruginosa from bronchoalveolar-lavage specimens obtained with use of endoscopy-suite bronchoscopes increased from 10.4 percent at base line to 31.0 percent during the outbreak (relative risk, 2.97; 95 percent confidence interval, 2.28 to 3.90). Cultures of samples from three bronchoscopes grew P. aeruginosa, where-as cultures of samples from the environment, instrument-cleaning machines, and gastrointestinal endoscopes did not. The three bronchoscopes had been part of a nationwide recall. A total of 414 patients underwent bronchoscopy during the outbreak, and there were 48 respiratory tract and bloodstream infections among 39 of these patients (9.4 percent). In 32 infections (66.7 percent), P. aeruginosa was confirmed as a potentially causative organism. Exposure to a potentially contaminated bronchoscope may have had a role in the death of three patients. The rate of recovery ofP. aeruginosa returned to base line after the instruments were removed from service. CONCLUSIONS: This large outbreak of P. aeruginosa infections related to bronchoscopy was apparently caused by a loose biopsy-port cap in the bronchoscopes. Instrument safety and surveillance methods for bronchoscopy must be improved, and better recall procedures are needed for medical devices.