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BACKGROUND/OBJECTIVES: Delirium is common in older adults, especially following hospitalization. Because low vitamin D levels may be associated with increased delirium risk, we aimed to determine the prognostic value of blood vitamin D levels, extending our previous genetic analyses of this relationship. DESIGN: Prospective cohort analysis. SETTING: Community‐based cohort study of adults from 22 cities across the United Kingdom (the UK Biobank). PARTICIPANTS: Adults aged 60 and older by the end of follow‐up in the linked hospital inpatient admissions data, up to 14 years after baseline (n = 351,320). MEASUREMENTS: At baseline, serum vitamin D (25‐OH‐D) levels were measured. We used time‐to‐event models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for the association between vitamin D deficiency and incident hospital‐diagnosed delirium, adjusted for age, sex, assessment month, assessment center, and ethnicity. We performed Mendelian randomization genetic analysis in European participants to further investigate vitamin D and delirium risk. RESULTS: A total of 3,634 (1.03%) participants had at least one incident hospital‐diagnosed delirium episode. Vitamin D deficiency (<25 nmol/L) predicted a large incidence in delirium (HR = 2.49; 95% CI = 2.24–2.76; P = 3*10−68, compared with >50 nmol/L). Increased risk was not limited to the deficient group: insufficient levels (25–50 nmol/L) were also at increased risk (HR = 1.38; 95% CI = 1.28–1.49; P = 4*10−18). The association was independent of calcium levels, hospital‐diagnosed fractures, dementia, and other relevant cofactors. In genetic analysis, participants carrying more vitamin D–increasing variants had a reduced likelihood of incident delirium diagnosis (HR =.80 per standard deviation increase in genetically instrumented vitamin D:.73–.87; P = 2*10−7). CONCLUSION: Progressively lower vitamin D levels predicted increased risks of incident hospital‐diagnosed delirium, and genetic evidence supports a shared causal pathway. Because low vitamin D levels are simple to detect and inexpensive and safe to correct, an intervention trial to confirm these results is urgently needed. [ABSTRACT FROM AUTHOR]