The clinical significance of des-gamma-carboxy prothrombin (PIVKA-II) in hepatocellular carcinoma (HCC) was investigated in 112 patients with and without vitamin K administration. The positivity rate of PIVKA-II was significantly decreased in patients receiving vitamin K (28.5%), compared with those without vitamin K administration (54.5%, p less than 0.05). The plasma levels of vitamin K derivatives [phylloquinone (VK1), menaquinone-4 (MK4), and menaquinone-7 (MK7)] measured were not decreased in patients with HCC, but were significantly increased in MK4 and VK1 + MK4 + MK7. The amount of PIVKA-II in plasma did not correlate with the plasma levels of vitamin K derivatives. However, PIVKA-II was decreased by the administration of vitamin K, and all of the six patients with more than 5.0 ng/ml of VK1 + MK4 + MK7 were within normal limits, whereas half of 32 patients with less than that had abnormal levels of PIVKA-II. Thus, it was suggested that PIVKA-II was not elevated due to vitamin K deficiency, but might result from the impaired metabolism or availability of vitamin K in the tumor. Therefore, PIVKA-II should be measured without vitamin K administration.