We measured menaquinone-4 (MK-4) and MK-4 epoxide concentrations in plasma and liver tissue after intravenous injection of 200 micrograms/kg MK-4 in 42 patients who underwent hepatectomy. They were classified into normal (N; n = 10), chronic hepatitis (CH; n = 12), and liver cirrhosis (LC; n = 20) groups, on the basis of the diagnosis given by the pathologist after examining resected liver specimens. The plasma MK-4 epoxide concentration reached maximum level (Cmax) 60 min after MK-4 injection. The Cmax in groups LC and CH were 85.9 and 126.3 nmol/l, respectively, which is significantly reduced compared with that of group N (184.4 nmol/l) (p less than 0.01 and p less than 0.05, respectively). The MK-4 concentrations in liver tissues of 24 patients 60 min after MK-4 injection were 2.77 in group N, 3.79 in group CH, and 3.83 nmol/g in group LC, and the MK-4 epoxide concentrations were 4.01, 3.09, and 2.62 nmol/g in the respective groups. Consequently, the ratio of MK-4 epoxide to total MK-4 (MK-4 + MK-4 epoxide) in groups CH and LC was significantly lower than in group N (p less than 0.01). It is concluded that the Cmax of MK-4 epoxide after MK-4 injection may serve as an indicator of liver function and that the low ratio of MK-4 epoxide to total MK-4 in the liver shows impairment in vitamin K metabolism.