Shea MK, et al. - This article was written with the objective to assess the relationship of vitamin K and D sufficiency with lower-extremity function in the Health, Aging Body Composition Knee OA Sub-study (Health ABC) and conducted a replication analysis in an independent cohort, the Osteoarthritis Initiative (OAI). It was concluded that the sufficient vitamin K status combined with sufficient vitamin D status was related to the better lower-extremity function in two knee OA cohorts. These outcomes merit confirmation in vitamin K and D co-supplementation trials.