This study was designed to evaluate the usefulness of Coenzyme Q10 (CoQ10) in the prevention of side effects due to anthracycline agents-Adriamycin (ADM) and Daunorubicin (DNR)-by comparing the preventive effect between CoQ10-treated and non-treated groups. The subjects were 79 patients, 55 of whom had malignant lymphoma. The age range was from 16 to 77 years with a mean age of 45.4 years. CoQ10 was administered by intravenous drip at 1 mg/kg/day the day before ADM or DNR administration, on the day and for a further 2 days after administration. In mean total dose, complete remission rate and mortality, no significant differences were observed between the 2 groups. Although there were also no significant differences in the degree of alopecia, fever, nausea and vomiting, the incidences of diarrhea and stomatitis were significantly (p less than 0.10 and p less than 0.05, respectively) reduced in the CoQ10-treated group. Depression of ST waves (more than 0.05 mV) and changes in T waves (R/10 greater than T, flat, inversion) on ECG were regarded as a parameter of aggravation. Such ECG aggravation was found in 20 of 40 patients given CoQ10 (50.0%) and in 18 of 25 receiving none (72.0%); a cardiotoxicity-inhibiting tendency was thus evident (p less than 0.10). In heart rate, tachycardia was noted in the nontreated group when the period of use of anthracycline agents exceeded 8 weeks. Twenty nine patients received ADM or DNR for 8 weeks or more, and, of them, 17 were treated with CoQ10; 11 of the 17 (64.7%) showed ECG aggravation, while 11 of 12 patients (91.7%) not treated with CoQ10 showed ECG aggravation. A tendency to depress ECG aggravation was thus observed in the treated group (p less than 0.10).