BACKGROUND AND AIMS: Vitamin K insufficiency is common and linked to an increased risk of cardiovascular disease and osteoporotic fractures. The aim of this study was to examine whether daily supplementation with oral vitamin K could improve vascular health and physical function in older people with established vascular disease. METHODS AND RESULTS: A double blind, randomised, placebo-controlled trial. Participants aged ≤ 70 years with a history of vascular disease were randomised to receive 6 months of daily oral 100mcg vitamin K2 (MK7 subtype) or matching placebo with outcomes measured at 0, 3 and 6 months. The primary outcome was between-group difference in endothelial function assessed using flow-mediated dilatation of the brachial artery at 6 months. Secondary outcomes included carotid-radial pulse wave velocity, augmentation index, blood pressure, carotid intima-media thickness, C-reactive protein, B-type natriuretic peptide, cholesterol and desphospho-uncarboxylated matrix Gla protein levels. Handgrip strength and the Short Physical Performance Battery assessed physical function, while postural sway was measured using a 3-dimensional force platform. RESULTS: 80 participants were randomised, mean age 77 (SD 5) years; 44/80 were male. Vitamin K levels rose in the intervention arm compared to placebo (+48 pg/ml vs −6 pg/ml, p=0.03) at 6 months. Desphospho-uncarboxylated Matrix Gla protein levels fell in the intervention group compared to placebo at 6 months (−130 SD 117 pmol/L vs +13 SD 180 pmol/L, p<0.001). No change was seen in endothelial function (between group difference −0.3% 95%CI −1.3 to 0.8, p=0.62). A modest, non-significant improvement in pulse wave velocity was seen in the vitamin K group (−0.8m/s 95%CI −1.8 to 0.3, p=0.15) while all other vascular and physical function outcomes unchanged. CONCLUSIONS: Six months of vitamin K2 supplementation did not improve markers of vascular health or physical function in older patients with vascular disease.