The purpose of the study was to investigate changes in serum ubiquinol/ubiquinone ratio with copper ion induced oxidative stress, and to assess the ubiquinol/ubiquinone ratio as marker of in vivo oxidative stress in patients with coronary artery disease (CAD). Plasma ubiquinol, ubiquinone, vitamin E (alpha-tocopherol) and total cholesterol (TC) concentrations were measured in 40 patients with angiographically confirmed coronary artery disease and 100 apparently healthy controls. The mean (SD) ubiquinol/ubiquinone ratio of 26.5 (7.5) of the CAD patients was significantly lower than the mean ratio of 30.2 (8.8) of the controls (p = 0.02). Our results indicate that the ubiquinol/ubiquinone ratio is a sensitive marker of oxidative stress and that an altered ubiquinol/ubiquinone ratio is the first sign of lipoprotein exposure to oxidative stress. The altered ratio in CAD patients cannot be explained by differences in plasma vitamin E levels. The vitamin E concentrations were in fact significantly higher in CAD patients, and did not appear to protect the ubiquinol from increased oxidation due to free radical reactions. These results may indicate that circulating lipoproteins of CAD patients are more exposed to, or are more susceptible to, free radical reactions compared with apparently healthy controls.