Several noninvasive studies have shown the effect on heart failure of treatment with coenzyme Q10. In order to confirm this by invasive methods we studied 22 patients with mean left ventricular (LV) ejection fraction 26%, mean LV internal diameter 71 mm and in NYHA class 2-3. The patients received coenzyme Q10 100 mg twice daily or placebo for 12 weeks in a randomized double-blinded placebo controlled investigation. Before and after the treatment period, a right heart catheterisation was done including a 3 minute exercise test. The stroke index at rest and work improved significantly, the pulmonary artery pressure at rest and work decreased (significantly at rest), and the pulmonary capillary wedge pressure at rest and work decreased (significantly at 1 min work). These results suggest improvement in LV performance. Patients with congestive heart failure may thus benefit from adjunctive treatment with coenzyme Q10.