The frequency of complications following flexible fiber-optic bronchoscopic procedures was studied prospectively in 908 patients from 13 cooperating hospitals. Major complications (respiratory arrest, pneumonia, pneumothorax, and obstruction of airways) occurred in 1.7 per cent (15) of the procedures. There was one death, yielding a mortality of 0.1%. Minor complications, including vasovagal reactions, fever, cardiac arrhythmias, bleeding, obstruction of airways, nausea and vomiting, pneumothorax, psychotic reaction, and aphonia, occurred in 6.5% of the procedures. Pneumothorax occurred after 5% (four) of 85 transbronchial biopsies. Although serious complications occur more frequently than previously reported from retrospective studies, complications after fiberoptic bronchoscopic procedures are still quite infrequent. The relative risks and benefits must always be carefully weighed in patients being considered for a fiberoptic bronchoscopic procedure.