In 60 children, aged between 1 month and 22 years (median 3.54 years) and with a body weight of 3-67 kg (median 12.6 kg), transvascular endomyocardial biopsy (EMB) was performed from the right (35 children) or left ventricle (30 children). The specimens were investigated by light and electron microscopy. There were three indications for biopsy: (1) poorly functioning, dilated left ventricle (seven patients with endocardial fibroelastosis, 16 with dilated cardiomyopathy, six from healing/healed or chronic myocarditis); (2) unexplained left ventricular hypertrophy (10 with hypertrophic cardiomyopathy, four with secondary hypertrophy, three with storage diseases); (3) to answer certain questions in eight children (four with hypoxic and two with cytoxic myocardial damage). Retrospectively, there were five nonindicated biopsies. There were no serious complications. Biopsies were diagnostic in 11.7% of cases, helpful in 71.7%, and of no help in 16.6%. Thus even in childhood endomyocardial biopsy is a diagnostic tool which can add useful information on the etiology or pathogenesis of an underlying myocardial disease.