Objectives: The primary objective was to explore the possible association of serum 25-hydroxyvitamin D (25[OH] D) and vitamin D intake with markers of glucose metabolism, depression, and cognitive performance. In addition, we examined to what extent the associations between vitamin D and cognitive performance were modified or mediated by fasting plasma glucose (FPG) levels. Design, Setting, and Participants: Cross-sectional study using data of 127 frail or prefrail Dutch elderly, aged 65 years or older. Frailty was defined according to the criteria of Fried and colleagues. A participant was classified prefrail when 1 to 2 criteria were met; frailty was classified as the presence of 3 or more criteria. Measurements: Associations of 25(OH) D and vitamin D intake with markers of glucose metabolism and domain-specific cognitive performance were examined by multivariable regression analyses. The possible association of vitamin D with depression and global cognitive performance was explored by Poisson regression. Results: No associations were observed for 25(OH) D with FPG, fasting plasma insulin (FPI), Homeostasis Model Assessment-estimated Insulin Resistance (HOMA-IR), or depression. In contrast, serum 25(OH) D was positively associated with executive functioning (beta 0.007, P=.01) and tended to be associated with information-processing speed (beta 0.006, P=.06). FPG did not modify or mediate these associations. Vitamin D intake was not associated with cognitive performance, glucose metabolism, or depression. Conclusion: This cross-sectional study suggests an association of serum 25(OH) D with domain-specific cognitive performance, in particular executive functioning and possibly information-processing speed, but not with FPG, FPI, HOMA-IR, or depression. Whether these associations are causal is yet to be demonstrated.