There are obviously several causes of myocardial dysfunction but energy deficiency of the myocytes may play a significant role and probably is a common mechanism during the progression of myocardial failure. Theoretically, a poor utilization efficiency of oxygen may be due to exhaustion of the myocardial stores of bioenergetics. In this report the authors review their biochemical results from measurements of coenzyme Q10 (CoQ10) levels in blood and human endomyocardial biopsies using an HPLC method from patients with suspected myocardial disease (n = 45). The levels of CoQ10, which has a key role in the respiratory chain and the synthesis of ATP, was found to be significantly decreased in various groups of patients with myocardial failure (dilated and restrictive cardiomyopathy and alcoholic heart disease) as compared to "normal" myocardium (0.42 +/- 0.04 micrograms/mg dry weight). The deficiency of CoQ10 was more pronounced with increasing symptoms; e.g. patients with dilated cardiomyopathy in NYHA Classes III and IV had lower tissue CoQ10 content than those of Classes I and II (0.28 +/- 0.04 vs. 0.37 +/- 0.06 micrograms/mg, p less than 0.001). Nearly two thirds of a series of 40 patients in severe heart failure (Classes III and IV) treated with CoQ10, 100 mg daily, in an open, controlled design showed subjective and objective improvement. Clinical responders were 69% and 43% of patients with cardiomyopathy and ischaemic heart disease, respectively. The results suggest that CoQ10 is a novel and effective breakthrough in heart-failure therapy and it appears safe, as no adverse reactions were registered. The through in heart-failure therapy and it appears safe, as no adverse reactions were registered.(ABSTRACT TRUNCATED AT 250 WORDS)