Aim. Des-gamma-carboxy prothrombin (DCP) is an abnormal prothrombin, increased in serum of patients with hepatocellular carcinoma (HCC) as result of an acquired defect of post-translational carboxylation of prothrombin's precursor. It is unclear if the reduced activity of gamma-carboxylase is secondary to vitamin K deficiency or to an altered gene encoding this enzyme. The aim of this study was to evaluate the effect of vitamin K administration on DCP and alpha-fetoprotein (AFP) levels, to identify a relationship between vitamin K and DCP serum levels and to investigate mechanisms of serum elevation of DCP levels. Methods. The authors determined DCP and AFP serum levels and vitamin K concentration in 64 cirrhotics with HCC and in 60 cirrhotic subjects without HCC. In HCC subjects DCP and AFP levels were measured before and after vitamin K administration. A t-test for unpaired data was applied (P values <0.05 statistically significant). Results. Only HCC patients had detectable levels of DCP and significant AFP levels. Administration of vitamin K reduced DCP but not AFP levels in HCC patients. No correlation was observed between vitamin K concentration and DCP levels: vitamin K concentration was similar both in HCC patients and in control group without HCC; HCC patients had the same vitamin K concentration regardless of elevated o reduced DCP levels after vitamin K administration. Conclusion. DCP detectable serum levels are the result not only of vitamin K deficiency or selective defects of carboxylase, because probably alterations of membrane receptors or cytoplasmatic transfers, that are necessary for the function of vitamin K, are involved.