An 85-year-old man with HCV infection and diabetes mellitus was diagnosed as having hepatocellular carcinoma (HCC, 13 cm in diameter) based on high serum alpha-fetoprotein (AFP), AFP-L3, and des-y-carboxy pro thrombin levels as well as typical enhancement pattern on contrast-enhanced CT. The patient did not receive any interventional treatments because of advanced age and the advanced stage of HCC. He chose to take vitamin K, which was reported to suppress the growth of HCC in vitro. Three months after starting vitamin K, all three tumor markers were normalized and HCC was markedly regressed, showing no enhancement in the early arterial phase on CT. Here we present the report describing the regression of HCC during the administration of vitamin K. An 85-year-old man with HCV infection and diabetes mellitus was diagnosed as having hepatocellular carcinoma (HCC, 13 cm in diameter) based on high serum alpha-fetoprotein (AFP), AFP-L3, and des-y-carboxy pro thrombin levels as well as typical enhancement pattern on contrast-enhanced CT. The patient did not receive any interventional treatment because of advanced age and the advanced stage of HCC. He chose to take vitamin K, which was reported to suppress the growth of HCC in vitro Three months after starting vitamin K, all three tumor markers were normalized and HCC was markedly regressed, showing no enhancement in the early arterial phase on CT. Here we present the report describing the regression of HCC during the administration of vitamin K.