Background.Recent European studies suggest that vitamin D deficiency may be associated with increased odds of cognitive impairment in older persons, although findings from the United States are equivocal. Our objective was to investigate the association between vitamin D deficiency and cognitive impairment in the elderly U.S. population. Methods.Three thousand and three hundred twenty-five adults aged 65 years or more completed cognitive assessments, medical examinations, and physical performance measures and provided blood samples in the Third National Health and Nutrition Examination Survey, a nationally representative cross-sectional study of the U.S. noninstitutionalized population. We determined whether low levels of serum 25-hydroxyvitamin D (25(OH)D) were associated with increased odds of cognitive impairment using logistic regression models. Cognitive impairment was assessed using measures of immediate and delayed verbal memory, orientation, and attention (impairment was defined as the worst 10% of the distribution of combined scores). Results.The multivariate adjusted odds ratios (95% confidence interval) of cognitive impairment in participants who were 25(OH)D insufficient ([≥]50 < 75 nmol/L), deficient ([≥]25 < 50 nmol/L), and severely deficient (<25 nmol/L) in comparison with those sufficient ([≥]75 nmol/L) were 0.9 (0.6-1.3), 1.4 (1.0-2.1), and 3.9 (1.5-10.4), respectively (p for linear trend = .02). Log-transformed levels of 25(OH)D were also significantly associated with the odds of cognitive impairment (p = .02). Conclusions.These findings suggest that vitamin D deficiency is associated with increased odds of cognitive impairment in the elderly U.S. population. Further exploration of a possible causal relationship between vitamin D deficiency and cognitive impairment is warranted.