We have reported the development of 1.6 diopters of myopia in a group of patients exposed to reptitive oxygen breathing at two atmospheres absolute pressure. No significant change in corneal curvature accompanied this refractive change. During the three months following termination of the hyperoxic exposure, the myopia gradually disappeared. It is speculated that hyperoxygenation alters the metabolism of the adult lens producing an increase in its dioptric power.