An outbreak of Pseudomonas aeruginosa infections occurred after thoracic surgeries performed between May and June 2003. Clinical data of seven patients were reviewed and the fact was revealed that bronchoscopes were used during endotracheal intubation for one-lung ventilation in most patients. P. aeruginosa was recovered from the sputum of these patients at a very early stage post-operation. Environmental samples from bronchoscopes and an automated endoscope reprocessor (AER) were cultured and P. aeruginosa strains were recovered from all of them. All of these strains were confirmed to be identical by pulsed-field gel electrophoresis (PFGE). Inspection of the sterilization cycles of bronchoscopes revealed inappropriate management of bronchoscopes and a flaw in the AER; once its detergent tank was contaminated, it was not possible to disinfect it. After all the bronchoscopes had been disinfected, and the washing machine had been remodeled, with the washing process confirmed to be appropriate, the outbreak finally ended. This outbreak had two causes, a flaw in the AER and inappropriate disinfection procedures. Outbreaks associated with bronchoscopic examinations have been reported elsewhere. Bronchoscopes are widely used to facilitate endotracheal intubation, especially for one-lung anesthesia. Although they are used for only a short time during anesthetic procedures, we should handle them more carefully.