Serum undercarboxylated osteocalcin (s-ucOC) is a marker for vitamin K metabolism (deficiency). The aim of this study was to investigate the serum levels of ucOC in patients with bilateral knee osteoarthritis (K-OA), and the correlation between ucOC and other biomarkers for K-OA. A total of 25 patients (22 women, 3 men, mean age 76.0±7.8, range 54-88 years, mean BMI 24.9±4.7) with a Kellgren-Lawrence grade of 3 or 4 for bilateral knee were enrolled in this study. The levels of s-ucOC and other biomarkers were measured. The levels of s-ucOC (5.66±4.70 ng/ml) as well as other cartilage metabolism markers, were elevated in the patients; however, bone metabolism markers were within the normal ranges. Of interest, there was a significant correlation between s-ucOC and serum hyaluronan (a marker for synovitis) (P<0.05). Our findings suggest that vitamin K metabolism may be associated with synovitis in patients with K-OA, and s-ucOC could be a biomarker for K-OA.