Technical Abstract: Matrix Gla protein (MGP) is a calcification inhibitor in vascular tissue. To function, MGP must be carboxylated by vitamin K. Evidence suggests that circulating uncarboxylated MGP (ucMGP) is elevated in diseased individuals with vascular calcification. The extent to which this reflects vitamin K’s role in calcification is unknown. The purpose of this study was to determine cross-sectional and longitudinal associations between plasma ucMGP, vitamin K status and coronary artery calcium (CAC) in older adults free of coronary heart disease. Cross-sectional associations between baseline plasma ucMGP, vitamin K status biomarkers [plasma phylloquinone, uncarboxylated prothrombin (PIVKA-II), serum uncarboxylated osteocalcin (%ucOC)], and CAC were examined in community-dwelling adults (n=438, 60-80 yrs old, 59% women). The effect of phylloquinone supplementation (500 ug/day, 3 years) on plasma ucMGP was determined among 374 participants. At baseline plasma phylloquinone was lower, and %ucOC and PIVKA-II were higher across increasing quartiles of plasma ucMGP (all p<0.001, age-adjusted). Plasma ucMGP was significantly reduced among the 190 participants who received phylloquinone supplementation compared to the 184 who did not [mean±SE change= -345 +/- 251pmol/L vs -40 +/- 250pmol/L, p<0.0001)]. After age adjustment, CAC did not differ according to ucMGP quartile (p=0.35). In the phylloquinone supplementation group the change ucMGP was not associated with the change in CAC [unstandard B(SE) =-0.02(0.02), p=0.44]. Plasma ucMGP was associated with known biomarkers of vitamin K status and was reduced following phylloquinone supplementation, suggesting it may be a useful marker of vitamin K status in vascular tissue. However, plasma ucMGP did not appear to reflect CAC in healthy older adults.