The objective of this study was to assess the effect of coenzyme Q10 (CoQ10) as supplementation to conventional antifailure drugs on quality of life and cardiac function in children with chronic heart failure due to dilated cardiomyopathy (DCM). The study was an open-label prospective study performed in two of the largest pediatric centers in Thailand from August 2000 to June 2003. A total of 15 patients with idiopathic chronic DCM were included, with the median age of 4.4 years (range, 0.6-16.3). Presenting symptoms were congestive heart failure in 12 cases (80%), cardiogenic shock in 2 cases (13.3%), and cardiac arrhythmia in 1 case (6.7%). Sixty-one percent of patients were in the New York Heart Association functional class 2 (NYHA 2), 31% in NYHA 3, and 8% in NYHA 4. Cardiothoracic ratio from chest x-ray, left ventricular ejection fraction, and left ventricular end diastolic dimension in echocardiogram were 0.62 (range, 0.55-0.78), 30% (range, 20-40), and 5.2 cm (range, 3.8-6.5), respectively. CoQ10 was given at a dosage of 3.1 ? 0.6 mg/kg/day for 9 months as a supplementation to a fixed amount of conventional antifailure drugs throughout the study. At follow-up periods of 1, 3, 6, and 9 months, NYHA functional class was significantly improved, as was CT ratio and QRS duration at 3 and 9 months follow-up with CoQ10 when compared to the baseline and post-discontinuation of CoQ10 at 9 months (range, 4.8-10.8). However, when multiple comparisons were taken into consideration, there was no statistical significant improvement. In addition to the conventional antifailure drugs, CoQ10 may improve NYHA class and CT ratio and shorten ventricular depolarization in children with chronic idiopathic DCM.