We have performed 2, 222 procedures of fiberoptic bronchoscopy (FBS) in 1985 for diagnostic or therapeutic purposes. A total of 797 bacteriological studies on bronchoalveolar lavage fluid (BALF) were undertaken, including smear tests and cultures for mycobacteria revealed a total of 86 cases of positive results for mycobacteria. We further checked and analyzed all these cases of mycobacteria-positive results and compared the results with each individual's clinical state, chest X-ray film, bronchoscopic findings, sputum bacteriology and FB scope (9 types) used, and discovered an unusually high incidence of mycobacteria false-positive results. Both fiberoptic bronchoscopes and automatic endoscope washers were found to be contaminated by atypical mycobacteria ; M. chelonae subsp. chelonae, M. chelonae subsp. abscessus, and M. gordonae. These species were frequently (73%) cultured from BALF samples of 15 cases, in which the subspecies was identified, out of 86 cases of mycobacteria-positive results, although routine sputum bacteriological examination results were negative.