African Americans have increased risk for cardiovascular and cerebral vascular disease relative to Caucasian Americans. While it is generally accepted that arteries become stiffer at a younger age in African Americans; less is known regarding cerebral vascular function / reactivity (CVMR) to hypercapnia in African Americans. Furthermore, little is known regarding the relationship between arterial stiffness and CVMR, particularly in young healthy adults. We hypothesized that African Americans have stiffer arteries (i.e. arterial stiffness) and reduced CVMR during hypercapnia relative to Caucasian Americans. We also hypothesized that there would be a negative relationship between arterial stiffness and CVMR. Lastly, we hypothesized that these responses would be related to a decrease in Vitamin D status in this population and there would be correlation between Vitamin D status and CVMR. In 11 African American and 19 Caucasian American subjects central arterial stiffness was indexed from carotid-femoral pulse wave velocity (PWV). CVMR was assessed by the cerebral vascular conductance (CVC) response to rebreathing-induced hypercapnia. Vitamin D status was assessed from plasma 25(OH) Vitamin D. PWV was elevated in the African Americans (African American: 581.16 ± 27.7 cm/sec vs. Caucasian American: 502.98 ± 17.6 cm/sec; P < 0.01). CVMR was significantly reduced during hypercapnic rebreathing in the African Americans (African American: 3.05 ± 0.38% of baseline/mmHg vs. Caucasian American: 5.09 ± 0.29% of baseline/mmHg; P < 0.001). When data from all subjects was included there was a trend towards a negative relationship (R = 0.32, P = 0.10) between PWV and CVMR. Vitamin D status was significantly lower in African Americans (African American: 14.96 ± 0.97 ng/ml vs. Caucasian American: 32.73 ± 0.99 ng/ml; P < 0.001); however, there was no significant relationship between Vitamin D status and CVMR (R = 0.23 P = 0.23). In conclusion, these data indicate that African Americans have impaired cerebral vascular responses to hypercapnia, stiffer arteries, and lower Vitamin D status when compared with Caucasian Americans. In addition, there may be a negative relationship between CVMR and PWV; however, no significant correlation between Vitamin D status and vascular function including PWV or CVMR was observed in this study. / text