An unique combination treatment for cancer patients has been attempted in our department. The treatment consists of 500 rad irradiation of cobalt 60 on the first day and drip infusion of mixture of 50mg adriamycin, 500mg cyclophosphamide and 500mg 5-fluorouracil on the next day. This combination therapy was repeated every 3 weeks. The myocardial intoxication may be a great problem in this therapy. Investigation was performed in 40 cancer patients in order to clarify of Coenzyme Q10 (CoQ10) could show any protecting effect upon the possible myocardial intoxication. All patients were divided into 2 groups; one with CoQ10 of 20 patients, who received CoQ10 of 90mg/day orally and the other without CoQ10 of 20 patients. In the group without CoQ10, cardiothoracic ratio (CTR) and pulse rate increased significantly in all patients and on ECG low voltage of QRS complex was seen in 2 cases, changes of ST-segment, T-wave and appearance of arrhythmia were more than frequent in the group without CoQ10 than that with CoQ10. It is concluded that CoQ10 is effective for protecting the myocardium in this cancer therapy.