The literature concerning the importance of coenzyme Q10 in health and disease has been reviewed. Usual dietary intake together with normal in vivo synthesis seems to fulfil the demands for Q10 in healthy individuals. The importance of Q10 supplementation for general health has not been investigated in controlled experiments. The literature allows no firm conclusions about the significance of Q10 in physical activity. In different cardiovascular diseases, including cardiomyopathy, relatively low levels of Q10 in myocardial tissue have been reported. Positive clinical and haemodynamic effects of oral Q10 supplementation have been observed in double-blind trials, especially in chronic heart failure. These effects should be further examined. No important adverse effects have been reported from experiments using daily supplements of up to 200 mg Q10 for 6-12 months and 100 mg daily for up to 6 y.