A fall of arterial blood oxygen pressure (Pao2) to 20 mm Hg or less has generally been held incompatible with survival, but there are exceptions. We found survival in extreme hypoxemia to occur with a rate of more than one out of 1,700 hospital patients and it is especially likely in the presence of circulatory collapse. Permanent physiological impairment could not be identified when these patients were followed up for many months. Neither the degree of hypoxemia nor survival was well correlated with hydrogen ion or carbon dioxide levels. Evidence of severe neurological impairment did not preclude survival, and the neurological signs were reversible in most instances. Blood oxygen studies often reveal derangements not revealed by pH and carbon dioxide studies, and a low oxygen level should not be considered a sign of hopelessness.