Serial arterial blood gas analyses were done in 18 patients before, during, and after diagnostic fiberoptic bronchoscopy. Patients with marked hypoxemia or hypercapnia were excluded. An average decline in arterial oxygen pressure (Po2) of 20 mm Hg was observed following the procedure, and the alveolar-arterial oxygen tension gradient during 100% oxygen breathing widened an average of 56 mm Hg. The decline in arterial Po2 lasted from one to more than four hours after the procedure. Only one patient developed hypercapnia. The mechanisms for the arterial Po2 decline remain speculative. However, the extent and duration of arterial Po2 decline observed in this "good risk" group suggest that special precautions should be observed to avoid hypoxemia in patients undergoing fiberoptic bronchoscopy, including monitoring of arterial blood gases and administration of oxygen during and following the procedure. (JAMA 230:1666-1667, 1974)