OBJECTIVE: To test the hypothesis that lower 25-hydroxyvitamin D [25(OH)D] levels are associated with a greater likelihood of cognitive impairment and risk of cognitive decline. METHODS: We measured 25(OH)D and assessed cognitive function using the Modified Mini-Mental State Examination (3MS) and Trail Making Test Part B (Trails B) in a cohort of 1,604 men enrolled in the Osteoporotic Fractures in Men Study and followed them for an average of 4.6 years for changes in cognitive function. RESULTS: In a model adjusted for age, season, and site, men with lower 25(OH)D levels seemed to have a higher odds of cognitive impairment, but the test for trend did not reach significance (impairment by 3MS: odds ratio [OR] 1.84, 95% confidence interval [CI] 0.81-4.19 for quartile [Q] 1; 1.41, 0.61-3.28 for Q2; and 1.18, 0.50-2.81 for Q3, compared with Q4 [referent group; p trend = 0.12]; and impairment by Trails B: OR 1.66, 95% CI 0.98-2.82 for Q1; 0.96, 0.54-1.69 for Q2; and 1.30, 0.76-2.22 for Q3, compared with Q4 [p trend = 0.12]). Adjustment for age and education further attenuated the relationships. There was a trend for an independent association between lower 25(OH)D levels and odds of cognitive decline by 3MS performance (multivariable OR 1.41, 95% CI 0.89-2.23 for Q1; 1.28, 0.84-1.95 for Q2; and 1.06, 0.70-1.62 for Q3, compared with Q4 [p = 0.10]), but no association with cognitive decline by Trails B. CONCLUSION: We found little evidence of independent associations between lower 25-hydroxyvitamin D level and baseline global and executive cognitive function or incident cognitive decline. © 2010 by AAN Enterprises, Inc.