We have applied our method for the simultaneous detection of plasma ubiquinol-10 (reduced form) and ubiquinone-10 (oxidized form) (S. Yamashita and Y. Yamamoto,Anal. Biochem.250, 66–73, 1997) to plasmas of normal subjects (n=16) and patients with chronic active hepatitis (n=28), liver cirrhosis (n=16), and hepatocellular carcinoma (n=20) to evaluate the pressure of oxidative stress in these patients. The average ubiquinone-10 percentages (± S.D.) in total ubiquinone-10 and ubiquinol-10 in the four groups were 6.4 ± 3.3, 12.9 ± 10.3, 10.6 ± 6.8, and 18.9 ± 11.1, respectively, indicating a significant increase in ubiquinone-10 percentage in patient groups in comparison to normal subjects. These results and a significant decrease in the plasma ascorbate level in patient groups indicate that oxidative stress is evident after the onset of hepatitis and the subsequent cirrhosis and liver cancer.