Right ventricular diagnostic biopsy was performed in 25 patients (12 females and 13 males) aged 19-48 (mean 32) with suspicion of acute or subacute myocarditis for definitive diagnosis verification and to determine indications for therapy with corticosteroids or azathioprine. 3-4 specimens taken from different sites of ventricle were estimated under the light microscope according to Dallas classification distinguishing active myocarditis with or without fibrosis from borderline myocarditis with inflammatory infiltration not causing myocytes damage. Routine clinical and laboratory tests, echocardiographic examination, ECG Holter monitoring and nuclear examination to evaluate left ventricular ejection fraction were performed in all patients. 6 patients underwent coronarography to exclude atherosclerotic changes in coronary arteries. Bioptic examination result confirmed myocarditis suspicion in all patients. Active inflammation was stated in 18 patients including 7 with coexisting fibrosis. Borderline myocarditis was stated in 7 cases. Comparison of clinical examination results with those of pathomorphologic study made possible to state, that tachycardia, cardiac murmur, arrhythmias and left ventricular function impairment were frequent observed in the group of active myocarditis. Ejection fraction less than 30% was observed only in patients with active myocarditis and mainly in those with coexisting fibrosis. Based on performed study we consider that myocardial biopsy allows not only to diagnose myocarditis but also is helpful for lesion extent assessment in myocardium with all clinical implications.