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Purpose: More time outdoors is associated with a lesser risk of myopia but the underlying mechanism is unclear. We tested the hypothesis that 25-hydroxyvitamin D (vitamin D) mediates the protective effects of time outdoors against myopia. Methods: We analyzed data for children participating in the Avon Longitudinal Study of Parents and Children (ALSPAC) population-based birth cohort: Non-cycloplegic autorefraction at age 7 to 15 years; maternal report of time outdoors at age 8 years and serum vitamin D2 and D3 at age 10 years. A survival analysis hazard ratio (HR) for incident myopia was calculated for children spending a high vs. low time outdoors, before and after controlling for vitamin D level (N=3,677). Results: Total vitamin D and D3, but not D2, levels were higher in children who spent more time outdoors [mean (95% CI) vitamin D in nmol/l: Total, 60.0 (59.4 to 60.6) vs. 56.9 (55.0 to 58.8), P=0.001; D3, 55.4 (54.9 to 56.0) vs. 53.0 (51.3 to 54.9), P=0.014; D2, 5.7 (5.5 to 5.8) vs. 5.4 (5.1 to 5.8), P=0.23]. In models including both time outdoors and sunlight-exposure-related vitamin D, there was no independent association between vitamin D and incident myopia [Total, HR=0.83 (0.66 to 1.04), P=0.11; D3, HR=0.89 (0.72 to 1.10), P=0.30], whilst time outdoors retained the same strong negative association with incident myopia as in unadjusted models [HR=0.69 (0.55 to 0.86), P=0.001]. Conclusions: Total vitamin D and vitamin D3 were biomarkers for time spent outdoors, however there was no evidence they were independently associated with future myopia.