An unusual increase in the frequency of isolation of Mycobacterium chelonae subspecies chelonae from specimens of bronchial washings was found between September and December 1992 in National Taiwan University Hospital. During this period, a total of 123 patients underwent fibreoptic bronchoscopy with an Olympus P20. Seventy six patients had bronchial washing for bacteriological study and cytological examination. Acid-fast bacilli were found in 21 patients, in 18 of whom Mycobacterium chelonae were isolated from bronchial washing cultures. Eight patients were treated as mycobacterial infected, because of the presence of unexplained pulmonary lesion, positive acid-fast stain and culture for Mycobacterium chelonae. Diagnosis of lung cancer was delayed in one patient because of the initial negative cytological study and positive bacterial culture. The fibreoptic bronchoscope was disinfected by automated washing machine (EW-20, Olympus) using 2.3% glutaraldehyde according to a standard protocol. From a survey to search for possible sources of contamination, they were identified at the suction channel of four different bronchoscopes. This episode proved to be a pseudoepidemic. The contamination was controlled by extensive suction and rinsing of the channel with 70% alcohol immediately after disinfection by the automated bronchoscope disinfection machine. This study shows that, despite using the disinfection machine, the suction channel could still be contaminated with Mycobacterium chelonae. This may cause diagnostic confusion and unnecessary antimycobacterial treatment.