Background: Recent studies have revealed a relationship between vitamin D deficiency and atherosclerosis. This study aims to investigate the impact of vitamin D deficiency and replacement on markers of subclinical atherosclerosis in young premenopausal women in whom vitamin D deficiency is prevalent. Methods: Thirty-one premenopausal vitamin D deficient women and 27 age and gender-matched control subjects were enrolled in this study. Markers of subclinical atherosclerosis including carotid intima-media thickness (cIMT), flow-mediated dilatation (FMD), endothelial progenitor cell (EPC) count and cytokine levels were determined at baseline. All measurements were repeated at 6-month follow-up in vitamin D-deficient subjects after vitamin D replacement. Results: Vitamin D deficient premenopausal women had lower FMD (9.9 +/- 1.3 vs. 13.8 +/- 1.7%, p < 0.001) and EPC counts at baseline. This population also had lower IL-10 and higher IL-17 levels. A 6-month vitamin D replacement therapy resulted in a significant increase in FMD (9.9 +/- 1.3 vs. 11.4 +/- 1.4%, p < 0.001) and EPC counts. Furthermore, cytokine profile shifted toward a more anti-inflammatory phenotype including elevated IL-10 and decreased IL-17 levels. cIMT was not different between patient and control groups and did not change following vitamin D replacement. Change in 25(OH) D and IL-17 levels were independent predictors of the change in FMD measurements following vitamin D replacement. Conclusion: This study demonstrates that endothelial function is impaired in otherwise healthy vitamin D deficient young premenopausal women and improves with 6-month replacement therapy. Immune-modulatory effects of vitamin D may, at least partly, be responsible for its beneficial effects on vascular health. (C) 2014 Elsevier Ireland Ltd. All rights reserved.