Background: The intensive care unit oxygen challenge is a routine screening test for donor lung function. An arterial Po2 of <300 mm Hg is generally considered evidence of inadequate pulmonary function that contraindicates the use of the lungs for transplantation. Methods: Between December 1, 2001 and January 24, 2004, the intensive care unit oxygen challenge was compared with the Po2 in samples drawn from each pulmonary vein confluence in the operating room in 91 donors (182 donor lungs) after the lungs were fully inflated (Fio2 1.0). There were 62 males and 29 females, with a median age of 27.5 years (13.6 to 55.4 years). Results: In 40 lungs, the pulmonary vein Po2 was >300 mm Hg, whereas the Po2 was <300 mm Hg on the last intensive care unit o2 challenge. The difference between the intensive care unit Po2 and the operating room pulmonary vein Po2 was greatest for donors with the lowest intensive care unit Po2. Of these 40 lungs, 8 were transplanted, all of whom had excellent graft function with a median intubation of 14.2 hours (6.1 to 23.8 hours). No patient sustained primary graft failure or an airway complication. Conclusions: The intensive care unit Po2 is an unreliable screening test for donor lung function, particularly when one lung is clear and the other is unclear radiographically. In this setting, intraoperative surgical assessment and pulmonary venous Po2 should be the basis for determining donor lung suitability. This strategy may provide an opportunity to increase donor lung availability. Copyright © 2005 by the International Society for Heart and Lung Transplantation.