Purpose: The a im of this study was to quantify ocular refractive changes after a standard hype rbaric oxygen (HBO) treatment protocol and to characterize the time period of re covery. Patients and Methods: Hyperbaric oxygen therapy was given for 90 min dai ly at a pressure of 240 kPa for 21 days. Oxygen was administered to 20 patients using an oronasal mask and to 12 patients using a hood. Follow-up examinations were carried out 2-4 days after treatment, and thereafter regularly for up to 1 0 weeks in both groups. Refraction was assessed automatically and by the monocul ar subjective refraction method. A subgroup of nine of the 20 patients to whom o xygen was administered by an oronasal mask underwent a separate eye examination, which included crystalline lens opacity measurements and LOCS III gradings. Resu lts: In the patients given oxygen by mask, there was a significant myopic shift in the mean spherical equivalent, which was largest 2-4 days after treatment. T he shift was-0.55 ±0.40 D in the right eye and-0.53 ±0.42 D in the left eye. In the patients given oxygen by hood, the largest shift was observed after 12- 16 days, and was-1.06 ±0.52 D in the right eye and-1.10 ±0.57 D in the left eye. The refractive changes returned to baseline 6 weeks and 10 weeks after HBO treatment, respectively. No significant changes in crystalline lens transparency were revealed. Conclusions: The myopic shift after HBO therapy recovers within 10 weeks and may be more pronounced when patients are given oxygen using a hood compared with using an oronasal mask. Purpose: The a im of this study was to quantify ocular refractive changes after a standard hype rbaric oxygen (HBO) treatment protocol and to characterize the time period of re covery. Patients and Methods: Hyperbaric oxygen therapy was given for 90 min dai ly at a pressure of 240 kPa for 21 days. Oxygen was administered to 20 patients using an oronasal mask and to 12 patients using a hood. Follow-up examinations were carried out 2-4 days after treatment, and yet regularly for up to 1 0 weeks in both groups. Refraction was assessed automatically and by the monocul ar subjective refraction method. A subgroup of nine of the 20 patients to whom o xygen was administered by or or nasal mask underwent a separate eye examination, which included crystalline lens opacity measurements and LOCS III gradings. Resu lts: In the patients given oxygen by mask, there was a significant myopic shift in the mean spherical equivalent, which was largest 2-4 days after treatment. T he shift was-0.55 0.40 D in the right eye and-0.53 ± 0.42 D in the left eye. In the patients given oxygen by hood, the largest shift was observed after 12- 16 days, and was-1.06 ± 0.52 D in the right eye and-1.10 ± 0.57 D in the left eye. The refractive changes returned to the baseline for 6 weeks and 10 weeks after HBO treatment, respectively. No significant changes in crystalline lens transparency were revealed. Conclusions: The myopic shift after HBO therapy recovers within 10 weeks and may be more pronounced when patients are given oxygen using a hood compared with using an oronasal mask.