To examine visual acuity, refraction and ocular status before, during and after hyperbaric oxygen therapy (HBOT). Twenty-nine patients underwent 40 standard protocol HBOT treatments. In all subjects, refraction and best corrected visual acuity were measured at baseline and after ten, 20, 30 and 40 treatments, and at a 12-week follow-up. A subgroup (n = 19) were given additional examinations at baseline, after 40 treatments and at follow-up including measuring central corneal thickness (CCT), corneal curvature, anterior chamber depth, lens thickness, axial length, fundus morphology, blood pressure and intraocular pressure (IOP). Lens colour and opalescence were graded using the Lens Opacities Classification System III (LOCS III). Myopic shifts [≥0.5 dioptre (D) spherical equivalent (SE)] occurred in 45 (77.6%) eyes. Median refractive changes of -0.75 D SE (right eye; p < 0.001) and -0.66 D SE (left eye; p < 0.001) were observed between pretreatment and treatment end (Wilcoxon signed rank test). Refraction returned to baseline at follow-up, except for a small persisting change towards myopia, median -0.25 D SE (left eye; p = 0.01). Using the LOCS III, median increases in lens nuclear colour, of 0.6 (right eye; p < 0.001) and 0.7 (left eye; p < 0.001), and opalescence of 0.7 (both eyes; p = 0.01) were found at the last examination. Small reductions were noted in CCT of -6.00 μm (right eye; p = 0.03) and -4.00 μm (left eye; p = 0.03), and IOP of -1.50 mmHg (left eye; p = 0.01). The transient myopic shift may have been due to lenticular refractive index changes. Reduced lens transparency is a potential consequence of HBOT. Riedl, Škiljić, Arnell, Wannholt, Zetterberg, Andersson Grönlund (2019). Myopic shift and lens turbidity following hyperbaric oxygen therapy – a prospective, longitudinal, observational cohort study. Acta ophthalmologica, 2019 Jan;():. https://www.ncbi.nlm.nih.gov/pubmed/30690920