Idiopathic dilated cardiomyopathy (IDCM) is a primary myocardial disease of unknown cause characterized by ventricular chamber enlargement with impaired contractile function. In familial forms of IDCM, mutations of genes coding for cytoskeletal proteins related to force transmission, such as dystrophin, cardiac actin, desmin, and delta-sarcoglycan, have been identified. Here, we report the data of a retrospective investigation carried out to evaluate the expression of atrial natriuretic peptide (ANP), CD34, troponin T and nestin in the myocardium of patients affected with IDCM. Formalin-fixed and paraffin-embedded consecutive tissue sections from the ventricular wall of 10 human normal hearts (NH) following forensic autopsy and 22 IDCM (living explanted hearts) were studied using primary monoclonal antibodies against ANP, CD34, troponin T and nestin by immunohistochemistry. Myocardial fibers were counted independently by three pathologists. Statistics included analysis of variance, log-rank test for Kaplan-Meier analysis, and kappa assessment for intra- and inter-observer variability. ANP and CD34 were significantly overexpressed in IDCM compared to NH (p<0.05). Conversely, troponin T and nestin expression levels did not show significant variation. Inter-observer kappa statistics showed a value of 0.87 and intra-observer kappa statistics a value of 0.98. Evaluation of the marker distribution in the myocardium of patients with IDCM CD34 expression curve was similar to that of troponin T (p<0.0001), although two groups could be identified. Patients with a difference of more than 20 myocardial fibers in expression of CD34 and troponin T had a somewhat less favorable survival although the difference was not significant. The analysis of cells positive for troponin T resulted in a similar number of cardiac fibers between NH and IDCM. This is in agreement with cardiac enlargement present in IDCM, which is due to ventricular dilatation rather than increased number of myocytes. Moreover, the expression of nestin, a marker of activation of myocardial precursors, did not change either, and this may confirm that there are no hyperplastic phenomena in the IDCM pathogenesis. The increase in ANP-positive cells in IDCM could be a consequence of neurohormonal activation due to a decline in the impaired myocyte contractility. Furthermore, since it was already shown that ANP could be important in the control of vascular remodeling, we postulated that the increase in CD34-positive cells might be functionally correlated with the increase in ANP production. Differential expression of CD34 and troponin T might be used in future studies to evaluate their prognostic value.