A defective myocardial energy supply--due to lack of substrates and/or essential cofactors and a poor utilization efficiency of oxygen--may be a common final pathway in the progression of myocardial diseases of various etiologies. The vitamin-like essential substance coenzyme Q10, or ubiquinone, is a natural antioxidant and has a key role in oxidative phosphorylation. A biochemical rationale for using coenzyme Q10 as a therapy in heart disease was established years ago by Folkers and associates; however, this has been further strengthened by investigations of viable myocardial tissue from the author's series of 45 patients with various cardiomyopathies. Myocardial tissue levels of coenzyme Q10 determined by high-performance lipid chromatography were found to be significantly lower in patients with more advanced heart failure compared with those in the milder stages of heart failure. Furthermore, the myocardial tissue coenzyme Q10 deficiency might be restored significantly by oral supplementation in selected cases. In the author's open clinical protocol study with coenzyme Q10 therapy (100 mg daily) nearly two-thirds of patients revealed clinical improvement, most pronounced in those with dilated cardiomyopathy. Double-blind placebo-controlled trials have definitely confirmed that coenzyme Q10 has a place as adjunctive treatment in heart failure with beneficial effects on the clinical outcome, the patients' physical activity, and their quality of life. The positive results have been above and beyond the clinical status obtained from treatment with traditional principles--including angiotensin-converting enzyme inhibitors.