OBJECTIVE: We tested the hypothesis that low plasma concentrations of 25-hydroxyvitamin D are associated with increased risk of symptomatic ischemic stroke in the general population. METHODS: We measured plasma 25-hydroxyvitamin D in 10,170 individuals from the general population, the Copenhagen City Heart Study. During 21 years of follow-up, 1,256 and 164 persons developed ischemic and hemorrhagic stroke, respectively. In a meta-analysis of ischemic stroke, we included 10 studies, 58,384 participants, and 2,644 events. RESULTS: Stepwise decreasing plasma 25-hydroxyvitamin D concentrations were associated with stepwise increasing risk of ischemic stroke both as a function of seasonally adjusted percentile categories and as a function of clinical categories of 25-hydroxyvitamin D (p for trend ≤ 2 × 10(-3) ). In a Cox regression model comparing individuals with plasma 25-hydroxyvitamin D concentrations between the 1st and 4th percentiles to individuals with 25-hydroxyvitamin D concentrations between the 50th and 100th percentiles, multivariate adjusted hazard ratio of ischemic stroke was 1.82 (95% confidence interval, 1.41-2.34). Comparing individuals with clinical categories of severe vitamin D deficiency (