Background/Aims: Hyperbaric oxygen (HBO) therapy is used in the treatment of several disorders. Little is known about the effects of HBO treatment on corneal thickness. The aim of this study was to investigate the effect of HBO treatment on central corneal thickness. Methods: Thirty-two subjects (18 males and 14 females, mean age 57.3 8 16.7 years) undergoing HBO treatment were consecutively enrolled. The subjects were assigned into diabetic (n = 16) and nondiabetic groups (n = 16). Best-corrected visual acuity was recorded before HBO treatment, and anterior and posterior segment examinations were performed on a slit lamp biomicrosope. Central corneal thickness was measured with an ultrasonic pachymeter before and immediately after HBO treatment, which lasted 120 min at 2.4 atmospheres absolute with three 30-min oxygen and two 5-min air breathing periods. Results: HBO treatment did not change the central corneal thickness in diabetic subjects (547.6 8 34.5 vs. 548.6 8 34.6 m ; p = 0.606). In nondiabetic subjects, however, the central corneal thickness was significantly reduced after HBO treatment (576.5 8 34.8 vs. 569.0 8 34.8 m ; p ! 0.001). Conclusion: A single exposure to HBO treatment reduced the central corneal thickness in nondiabetic subjects but not in diabetic subjects. However, the change in central corneal thickness was minor.