This multicenter study evaluated the efficacy and tolerability of coenzyme Q10 in 1715 outpatients with chronic heart failure (New York Heart Association classes II and III), stabilized with standard therapy for 3 months. The patients were treated with coenzyme Q10 at a daily dose of 50 mg for 4 weeks, in addition to receiving conventional therapy. The efficacy of coenzyme Q10 was assessed by an open study that evaluated the improvement in clinical signs and symptoms of heart failure. After the baseline evaluation the subjects were seen on days 15 and 30. The intensity of signs and symptoms was assessed by a semiquantitative 4-point scale. Our results demonstrate that the administration of coenzyme Q10 in association with standard therapy improves dyspnea at rest, exertional dyspnea, palpitations, cyanosis, hepatomegaly, pulmonary rales, ankle edema, heart rate, and systolic and diastolic blood pressure in patients with stabilized heart failure. The rate of improvement and the low number of side effects in this large group of patients demonstrate that despite some methodological limitations in the study design and the short period of treatment (4 weeks) coenzyme Q10 given at a daily dose of 50 mg led to an improvement in the signs and symptoms of heart failure and in the quality of life.