Right ventricular endomyocardial biopsy was carried out in thirty three pa tients with undetermined form of Chagas' disease. Fragments obtained by this method were analysed under light microscopy with hematoxilin-eosin, and Masson trichromic stains. Thirteen (39.4%) pa tients showed normal myocardial fragments and twenty patients (60.6%) had them altered. Alterations included fiber degeneration, volume changes, intersti tial edema, inflammatory infiltrates and fibrosis. These data permit to conclude that only part of patients with this form of Chagas' disease have an incipient myocardial attack and that the alterations found in the fragments obtained are mild. The remaining patients would be either individuals with chagasic infection without cardiac disease or have spon taneous healing. This should be considered in the future treatment of the dis ease.