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Purpose. Quantifying adaptation to light distortion of subjects undergoing orthokeratology (OK) for myopia during the first month of treatment. Methods. Twenty-nine healthy volunteers (age: 22.34 ± 8.08 years) with mean spherical equivalent refractive error −2.10 ± 0.93D were evaluated at baseline and days 1, 7, 15, and 30 of OK treatment. Light distortion was determined using an experimental prototype. Corneal aberrations were derived from corneal topography for different pupil sizes. Contrast sensitivity function (CSF) was analyzed for frequencies of 1.50, 2.12, 3.00, 4.24, 6.00, 8.49, 12.00, 16.97, and 24.00 cpd under photopic conditions. Results. Average monocular values of all light distortion parameters measured increased significantly on day 1, returning to baseline after 1 week (P<0.05 in all cases). Spherical-like aberration stabilized on day 7 for all pupil diameters, while coma-like for smaller pupils only. CSF was significantly reduced on day 1 for all spatial frequencies except for 1.5 cpd, returning to baseline afterwards. Significant correlation was found between light distortion and contrast sensitivity for middle and high frequencies (P<0.05) after 15 days. Conclusion. Despite consistently increased levels of corneal aberrations, light distortion tends to return to baseline after one week of treatment, suggesting that neural adaptation is capable of overcoming optical quality degradation. [ABSTRACT FROM AUTHOR]