24 endomyocardial biopsies were performed in children aged 4-18 (x = 12.9) without any complications. The endomyocardial biopsy was performed in 12 patients with the heart failure of unknown origin (6 - restrictive heart disease, 4 - dilated cardiomyopathy with arrhythmias, 2-post-inflammatory dilated cardiomyopathy), in 5 patients with a clinical diagnosis of myocarditis, in 1 with the hypertrophic cardiomyopathy, in 2 cases of the dilated cardiomyopathy without heart failure and in 1 girl with the recurrent exudative pericarditis. Active myocarditis with fibrosis was stated in 2 cases of a restrictive heart disease, mild inflammatory state in 2 patients with a clinical diagnosis of the myocarditis and in 3 others with the dilated cardiomyopathy. All of 7 patients with the myocarditis underwent the immunosuppressive therapy. In the patient with endomyocardial fibrosis the result of left ventricular biopsy corresponded with angiocardiographic diagnosis. Non-specific changes in biopsies were stated in 2 children with the restrictive cardiomyopathy and in 5 with the dilated cardiomyopathy. Bioptic, morphologic lesions in patients with the dilated cardiomyopathy did not correlate with hemodynamic parameters of contractility. Biopsies were normal in 6 subjects. Endomyocardial biopsy influenced on making a decision of the therapy in 9 of 21 patients (42.8%). Diagnosis was verified in 7 patients (38.3%) basing on endomyocardial biopsy. In 18 of 24 subjects (75%) endomyocardial biopsy contributed to the interpretation of the disease pathology.