The association of hypoxemia and cardiac arrhythmias with fiberoptic bronchoscopy (FOB) is known. The effects of various methods of inserting the bronchoscope on blood oxygen tensions, the true prevalence of arrhythmias, and the clinical significance of PaO2 and rhythm changes are poorly defined. Arterial blood gas values, oxygen saturation and cardiac arrhythmias detected by 24-hour Holter monitoring were compared in 20 male patients undergoing either transoral or transnasal FOB. No significant difference in oxygen saturation was found; however, PaO2 values fluctuated widely and unpredictably and were significantly higher in the eight patients in the transnasal FOB group. No life-threatening arrhythmias were induced during bronchoscopy; however, because of minor arrhythmias such as sinus tachycardia, the overall frequency of arrhythmias rose from 60% to 65%. The incidence of arrhythmias in this study was similar to that reported during pulmonary function testing.