OBJECTIVE Pulse oximetry was used to measure arterial oxygen saturation and the extent of hypoxemia in pediatric patients undergoing FB. METHODS Arterial oxygen saturation was measured (1) prior to the procedure to provide a baseline value, (2) when the bronchoscope was positioned in the nasopharynx, and (3) when the bronchoscope was positioned in the mid-trachea. METHODS Fiberoptic bronchoscopy was performed in the Pediatric Special Care Unit or in the Pediatric Pulmonary Laboratory using an Olympus BF3C4 fiberoptic bronchoscope with a 3.5-mm outer diameter. METHODS Thirty-six children who underwent diagnostic or therapeutic bronchoscopy for a variety of reasons were evaluated. They ranged in age from 6 to 142 months; 20 were male and 16 were female. METHODS There were no interventions. RESULTS Of the 36 patients, 29 experienced a fall in SaO2 levels exceeding 5 percent of baseline values. The youngest age group, 6 to 12 months, showed the greatest drop in saturation as compared with the other groups. Desaturation was significantly increased by midtracheal FB. CONCLUSIONS A decline in arterial oxygen saturation that may be substantial in infants and children undergoing FB examination was frequently noted, especially in smaller infants and when the bronchoscope was positioned in the mid-trachea. Supplemental oxygen and a brisk procedure time will minimize the risk of dangerous hypoxia.