The frequencies of fever, parenchymal infiltration, and bacteremia were studied prospectively after 100 flexible fiberoptic bronchoscopies performed transnasally under topical anesthesia. Fever occurred after 16 per cent, and parenchymal infiltration, after 6 per cent of the procedures. Most complications were mild and transient; however, one patient developed rapidly progressive pneumonia and died. No organisms were isolated from cultures of blood drawn at the time of the procedure or during complications. The organisms most commonly isolated from the sputum of the patients who developed pneumonia were the aerobic and anaerobic bacteria normally found in the mouth. Isolation of a significant pathogen before the procedure did not predispose to development of a complication. Advanced age (greater than 60 years) and the endoscopic findings of abnormalities were significant predisposing factors. Bronchial brushing, but not bronchial biopsy or bronchial washing, was associated with significantly higher complication rates in patients with bronchial carcinomas than in those without a neoplasm. The mechanism of the fever and parenchymal infiltration is thought to be related to obstructive atelectasis and infection produced by organisms present in the airways at the time of the procedures.