Modern clinical use of supplemental oxygen supposes that: (1) exposure to F(IO)(2) < or = 60% is without adverse effects, (2) an individual at risk of developing arterial hypoxemia can be protected by administering high F(IO)(2), and (3) routine administration of supplemental oxygen is useful, harmless, and clinically indicated. There is now substantial evidence that none of those 3 suppositions are correct, and, on the contrary, supplemental oxygen is actually detrimental to many of the patients who receive it. Supplemental oxygen is much overused and its use should be limited to the few conditions and situations in which it is truly effective, useful, and non-detrimental.