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Some respiratory units routinely administer supplemental oxygen to all patients during flexible bronchoscopy, but other units give oxygen only to those who desaturate (arterial oxygen saturation [SaO2], < 90%). We performed a study to examine both the requirement for supplemental oxygen and the effect of IV midazolam therapy on oxygenation during flexible bronchoscopy for patients with a known FEV1.Data on the SaO2 of patients during flexible bronchoscopy were collected prospectively for all procedures performed in our respiratory unit for the period 1992 to 1997.There were 1,051 flexible bronchoscopy procedures performed in which the patient had a known FEV1 and was not receiving supplemental oxygen before the procedure. Supplemental oxygen was commenced during or immediately after the procedure in 151 cases (14.4%), while a further 101 cases (9.6%) had momentary desaturation (ie, < 20 s) not requiring oxygen therapy. The lower the FEV1, the greater the risk of significant desaturation and the need for supplemental oxygen (p < 0.0001) [supplemental O2 therapy was administered in 35% of cases if FEV1 < 1.0 L, in 14% of cases if FEV1 was 1.0 to 1.5 L, and in 7% of cases if FEV1 > 1.5 L]. The use of low-to-moderate doses of midazolam as sedation did not affect the probability of the occurrence of significant desaturation (p = 0.204).This study supports guidelines that suggest that all patients should be monitored by pulse oximetry during flexible bronchoscopy. Desaturation may occur at any FEV1 level even without sedation. The majority of our patients did not require routine oxygen supplementation, especially the group with an FEV1 above 1 L.