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The aim of this study was to evaluate a clinical procedure based on trial rigid gas-permeable (RGP) contact lenses fitting to assess the real pre-operative visual acuity (VA) of highly myopic eyes undergoing surgery with an implantable collamer lens.Highly myopic eyes with a spectacle visual acuity (SVA) of 0.2 logMAR (6/9.6 Snellen) or worse were enrolled. A trial RGP contact lens with high dioptric power was fitted before collamer lens implantation. The trial contact lens residual refractive error was corrected by spectacle over-refraction to estimate contact lens VA (CLVA). The three-month post-operative uncorrected vision and visual acuities were also measured. The safety and efficacy indices of collamer lens implantation were calculated based on pre-operative SVA and CLVA, respectively.The study enrolled 42 patients with 68 highly myopic eyes with a mean pre-operative spherical equivalent of -16.40 ± 4.43 D. The mean pre-operative SVA was 0.38 ± 0.19 logMAR. Pre-operative trial RGP contact lens fitting resulted in a significant improvement of VA (2.02 ± 1.05 lines), which was statistically insignificantly different from the VA with the implantable collamer lens (2.07 ± 0.8 lines). The safety index of collamer lens implantation was 1.70 ± 0.38 in reference to the pre-operative SVA but significantly decreased to 1.02 ± 0.17 in reference to CLVA. Similarly, the efficacy index of 1.17 ± 0.48 on SVA decreased to 0.71 ± 0.28 on CLVA.Trial RGP contact lens fitting was found to indicate the potential implantable collamer lens VA more accurately than spectacle refraction. Consideration should be given to the use a trial RGP contact lens as part of the pre-operative work-up prior to collamer lens implantation in order to provide an assessment of the patient's potential post-operative VA.