Objective　To investigate the ultrastructural pathological alterations of the microvasculature and nerve fibers in the endomyocardial biopsied specimens of the left ventricular myocardium obtained from patients with idiopathic dilated cardiomyopathy and chronic heart failure.Methods　Transmission electron microscopic observations of endomyocardial biopsied specimens of the left ventricular myocardium were carried out in 10 patients with idiopathic dilated cardiomyopathy and chronic heart failure.Results　Various degrees of ultrastructural pathological alterations in the microvessels and sympathetic nerves in the diseased myocardium were consistently demonstrated in idiopathic dilated cardiomyopathy. In addition, abnormal accumulation of collagen tissue and edematous fluid were often seen in the interspace between myocardial cells and nerve endings and capillaries.Conclusions　Based on the ultrastructural pathological findings in this study, we consider that all the structures forming the muscle cells and the tissues around them, namely the microvessels and nerves may participate in the pathological process in the course of dilated cardiomyopathy. The damage of microvasculature and sympathetic nerves resulting from the underlying disease processes are considered to be an important pathogenetic mechanism responsible for progressive development of myocardial degeneration and dysfunction throughout the course of the disease. It is hoped that our data may provide some insights into the understanding of the role of microcirculation and sympathetic nerves in the etiopathogenesis of dilated cardiomyopathy.