The authors have tried to study the therapeutic efficacy of coenzyme Q10 (CoQ10) in patients with dilated cardiomyopathy (DCM). In fact, CoQ10 has been shown to be deficient in myocardial tissue biopsies taken from DCM hearts, compared to normal hearts. Thirty patients with histological diagnosis of DCM were orally treated with CoQ10 (100 mg/die) for 2 months. Before and after treatment a clinical examination with determination of NYHA class and an echocardiographic examination with determination of ejection fraction (EF) and of telediastolic (TDV) and telesystolic (TSV) volumes were performed, and blood was drawn for plasma CoQ10 determination. In seven patients the pretreatment endomyocardial level of CoQ10 was also assayed. Seven patients left the study because of poor therapeutic compliance. In 47% of patients the clinical symptomatology regressed, with improvement of NYHA class. The EF improved from 0.31 +/- 0.09 to 0.37 +/- 0.11 (p less than 0.001). The TDV passed from 262.2 +/- 85 ml to 203.3 +/- 83 ml (p less than 0.05), and the TSV from 166.13 +/- 75 ml to 126.9 +/- 56 ml (ns). The CoQ10 plasmatic levels improved in 95% of the patients: from 0.74 +/- 0.37 micrograms/ml to 2.27 +/- 0.99 micrograms/ml (p +/- 0.0001). The CoQ10 myocardial levels did not show univocal values, but the patients with lower myocardial levels seemed to have a better therapeutic response. These data suggest that the CoQ10 deficiency in DCM may be reversible and that the therapeutic effects depend on the basal plasmatic and myocardial levels. Therapy with coenzyme Q10 may be considered to be an efficacious aid in the traditional treatment of chronic cardiac failure.