On the basis of reports of the antitumor effects of vitamin K on various cancers, we clinically investigated the suppressive effects of vitamin K2 on tumor recurrence after curative treatment for hepatocellular carcinoma (HCC). Our results showed that vitamin K2 administration significantly suppressed HCC recurrence. Our laboratory findings revealed that the inhibitory effect of vitamin K2 against HCC cell growth was generated by suppressing cyclin D1 expression through inhibition of NF-kappaB activation.