One hundred and forty-three cases of chronic, stable, non-secondary, non-hypertrophic cardiomyopathy, 98% of whom were in NYHA Classes III and IV, were given 100 mg of coenzyme Q10 orally in addition to their conventional medical programme in an open-label long-term study. Blood CoQ10 levels, clinical status, myocardial function and survival have been recorded now for almost 6 years. Mean control/CoQ10 levels of 0.85 micrograms/ml rose to 2 micrograms/ml in 3 months and remained stable at that level. Mean ejection fraction of 44% measured by systolic time interval analysis rose to 60% within 6 months and stabilized at that level with 84% of patients showing statistically significant improvement. Eighty-five percent of patients improved by one or two NYHA Classes. Survival figures were encouraging with an 11.1% mortality in 12 months and 17.8% mortality in 24 months, comparing favourably with several reports in the literature. There was no positive evidence of toxicity or intolerance in a total of 368.9 patient-years of exposure. Coenzyme Q10 is safe and effective long-term therapy for chronic cardiomyopathy.