BACKGROUND: Vitamin K modulates calcification by activating calcification inhibitors such as matrix Gla protein (MGP). In kidney transplant recipients, vitamin K insufficiency is common, but implications for long-term outcomes are unclear. STUDY DESIGN: Single-center observational study with a longitudinal design. SETTING & PARTICIPANTS: 518 stable kidney transplant recipients; 56% men; mean age, 51±12 (SD) years; and a median of 6 (IQR, 3-12) years after kidney transplantation. FACTOR: Plasma desphosphorylated-uncarboxylated MGP (dp-ucMGP) levels, reflecting vitamin K status. OUTCOMES: All-cause mortality and transplant failure. RESULTS: At inclusion, median dp-ucMGP level was 1,038 (IQR, 733-1,536) pmol/L, with 473 (91%) patients having vitamin K insufficiency (defined as dp-ucMGP>500pmol/L). During a median follow-up of 9.8 (IQR, 8.5-10.2) years, 152 (29%) patients died and 54 (10%) developed transplant failure. Patients in the highest quartile of dp-ucMGP were at considerably higher mortality risk compared with patients in the lowest quartile (HR, 3.10; 95% CI, 1.87-5.12; P for trend