30 patients in paediatric age have been submitted to endocardial biopsy of the right ventricle, most of them affected by a dilated cardiomyopathy. Our aim was to improve knowledge on this disease using ultrastructural morphometric analysis. An enlargement of myocardial cells with granular aspects of the cytoplasm, known as cloudy swelling, was observed at histological level. Electronmicroscopy showed two main abnormalities: 1) a constant increase of the chondroma resulting in nuclear deformation and myofibrillary dislocation towards the sarcolemma. 2) disarrangement of thin (actin) and thick (myosin) myofilaments. Such myofibrillary dislocation, designated as fibrillar segregation, suggests how myocardial fibres may loose their ability to contract. The stereological study of these two ultrastructural abnormalities showed that mitochondria are increased in number and size and that the loss of contractile properties of myocardial cells can be related to myofibrillar segregation.