Ubiquinone (CoQ10 coenzyme) is part of the respiratory chain in mitochondria, and acts as a scavenger in oxidative stress in cell membranes. Ubiquinone is mainly synthesized in the liver and partly derived from the diet; its plasma levels significantly correlate with tissue levels in experimental animals and in pathological states in man. By means of an original high-performance liquid chromatography technique, we measured ubiquinone plasma levels in 10 healthy subjects, in 27 patients with cirrhosis and in 22 chronic alcoholics with normal liver function. Ubiquinone levels were markedly reduced in cirrhosis (0.25 [SD 0.21] microgram/ml vs. 0.92 [0.38] in controls; P < 0.001), without any difference between alcohol- and non-alcohol-related disease. Also, in chronic alcoholics ubiquinone levels were nearly halved (0.49 [0.24]). In cirrhosis, ubiquinone plasma levels significantly correlated with cholesterol (P < 0.05), and with total bilirubin levels (P < 0.01). Our study highlights a remarkable deficiency in ubiquinone levels in patients with cirrhosis and in chronic alcoholics, to which both reduced hepatic synthesis and nutritional defects may contribute.