Coenzyme Q10 was discovered as an essential cofactor of mitochondrial ATP production in 1957. Coenzyme Q10 is ubiquitously present in other biomembranes and even in lipoproteins. Reduced form of coenzyme Q10 (ubiquinol-10) is believed to work as antioxidant in these places. When human plasma was incubated under aerobic conditions, ubiquinol-10 depleted after the depletion of ascorbate but α-tocopherol remained unchanged, indicating that ascorbate and ubiquinol-10 are front-line antioxidants against oxygen radicals. The redox status of plasma coenzyme Q10 is a good indicator of oxidative stress since oxidative stress is defined as a disturbance in the prooxidant-antioxidant balance in favor of the former. We therefore developed a simple and reliable method for the simultaneous detection of both oxidized and reduced forms of coenzyme Q10. We applied this method to patients plasma and demonstrated an increase of oxidative stress in patient's with hepatitis, cirrhosis, and hepatoma, in LEC rats (animal model of liver cancer), in newborn babies, in patients with Parkinson disease, and others.