The arguments in favour of mitochondrial pathology of certain childhood cardiomyopathies (multi tissue involvement, lactic acidosis, histochemical abnormalities of skeletal muscle) are indirect and may be absent in isolated cardiomyopathy. The authors therefore set up a prospective study of enzyme activity of the mitochondrial respiratory chain directly by endomyocardial biopsy. Fifty children aged 2 months to 16 years were included. Thirty four had cardiomyopathy which was dilated and hypokinetic with thin walls in 21 cases, restrictive in 2 cases, and hypertrophic in 11 cases; the remaining 16 children had either normal hearts (13 catheterised for other reasons) or myocardial hypertrophy due to pulmonary or aortic stenosis (3 cases). Both ventricles were evaluated in 3 cases; macro-surgical biopsies were obtained in 6 cases and skeletal muscle biopsy was obtained in 9 cases. The results indicate the method to be reliable with no difference between the micro and macro biopsies. The absolute values of enzyme activity were too variable to serve as quantitative parameters but some ratios of activity were remarkably stable and allowed a qualitative assessment which was all the more accurate when identical values were obtained in the myocardium, skeletal muscle and liver. The mitochondrial respiration was independent of ventricular pressures and of the type (right or left) of ventricle. Enzyme activity was nearly always normal in dilated cardiomyopathy (20/21) which suggests that it was unaffected by dilatation of the heart and by abnormal myocardial contractility. The results could be normal in myocardial hypertrophy and in valvular stenosis and in over half the cases of hypertrophic cardiomyopathy.(ABSTRACT TRUNCATED AT 250 WORDS)