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Observational studies have reported increased risk of ischemic stroke among individuals with low serum 25-hydroxyvitamin D (S-25OHD) concentrations but uncertainty remains about the causality of this association. We sought to determine whether S-25OHD concentrations are causally associated with ischemic stroke and its subtypes using Mendelian randomization.We used summary-level data for ischemic stroke (34 217 cases and 404 630 noncases) from the MEGASTROKE consortium. As instruments, we used 6 single nucleotide polymorphisms, explaining 7.5% of the variance in S-25OHD, previously identified to be associated with S-25OHD concentrations in the Study of Underlying Genetic Determinants of Vitamin D and Highly Related Traits consortium (n=79 366). The analyses were conducted using the inverse-variance–weighted method and complemented with the weighted median, heterogeneity-penalized, and Mendelian randomization-Egger approaches.Genetically higher S-25OHD concentration was not associated with ischemic stroke. The odds ratios (95% CI) per genetically predicted 1-SD (≈18 nmol/L) increase in S-25OHD concentrations, based on all 6 single nucleotide polymorphisms, were 1.01 (0.94–1.08; P=0.84) for all ischemic stroke, 0.94 (0.80–1.11; P=0.49) for large artery stroke, 0.95 (0.82–1.11; P=0.55) for small vessel stroke, and 1.02 (0.90–1.16; P=0.74) for cardioembolic stroke. The results were similar in sensitivity analyses.These findings provide no support that higher S-25OHD concentrations are causally associated with any ischemic stroke subtype. Thus, vitamin D supplementation will unlikely reduce the risk of ischemic stroke in the general population.