From May, 1978 through Oct., 1979 the effect of Coenzyme Q10 was evaluated on 10 patients with chronic heart failure. The subjects selected for this study consisted of 7 patients with cardiomyopathy and 3 patients with valvular heart disease admitted to Severance Hospital, Yonsei University College of Medicine, All these patients had symptoms and signs of congestive heart failure which was stable for at least one month before starting Coenzyme Q10. Coenzyme Q10 was administered 30mg daily for eight weeks, one hour before meal and in combination with digitalis and/or diuretics. The drug effects were determined by measuring the cardio-thoracic ratio by chest X-ray, the sum of 'S' wave in V1 and 'R' wave in V5 in electrocardiogram, and PEP/LVET (pre-ejection period/left ventricular ejection time) by simultaneous tracings of carotid pulse and phonocardiogram every two weeks during medication. The cardio-thoracic ratio was improved in 4 of 10 cases, the sum of RV+SV5 was decreased in all 5 cases who showed voltages above 40mm before medication, and PEP/LVET ratio was decresed in 4 of 10 cases. The difference of average values before and after medication were not statistically significant(p<0.05) in all 3 parameters when examined by t-test. During treatment, there was improvement, if any, from two weeks after medication and no significant side effects were noted throughout the study period.