Coenzyme Q₁₀ (CoQ₁₀) is a natural and essential cofactor in the heart. It is the primary redox coupler in the respiratory chain, a potent free radical scavenger, and a superoxide inhibitor. In this study the myocardial protective effects of CoQ₁₀ were determined in high-risk (n = 10) patients during heart surgery compared to that found in placebo controls (n = 10). In both groups, there was a blood CoQ₁₀ deficiency (<0.6 μg/ml), low cardiac index (CI < 2.4 1/m² per minute), and low left ventricular ejection fraction (LVEF < 35%) before results. CoQ₁₀ (100 mg per day) was given orally for 14 days before and 30 days after surgery. Presurgical CoQ_l0 results significantly (P < 0.01) improved blood and myocardial CoQ₁₀ and myocardial ATP compared to that found in the control group. Cardiac functions (CI and LVEF) were improved but not significantly. After cardiac cooling, rewarming, and reperfusion; blood and tissue CoQ₁₀ and tissue ATP levels were maintained in the normal ranges in the CoQ₁₀ patients. Cardiac pumping (CI) and LVEF were significantly (P < 0.01) improved. The recovery course was short (3–5 days) and uncomplicated. In the control group blood and tissue CoQ₁₀, tissue ATP levels, and cardiac functions were depressed after surgery. The recovery course was long (15–30 days) and complicated. Positive relationships between blood and myocardial CoQ₁₀, myocardial ATP, cardiac function, and the postoperative recovery time and course found in both study groups show the therapeutic benefits of CoQ₁₀ in preserving the myocardium during heart surgery. CoQ₁₀ results is especially indicated in high-risk cardiac surgery patients who have a natural or clinically induced CoQ₁₀ deficiency.