Objective: Although chronic myocardial inflammatory process mediated by viral and autoimmune factors has been postulated in the pathogenesis of idiopathic dilated cardiomyopathy (IDC), the role of autoimmune mechanisms still remains unclear. The aim of the present study was to investigate the rates of various T cell subsets and natural killer (NK) cells in peripheral blood in order to see whether they had a role in the immunoregulation of IDC. Materials and Methods: The surface markers of peripheral T and B lymphocytes were detected and percentages of pan T and B cells as well as helper (CD4+) and suppressor (CD8+) T lymphocytes subsets in the peripheral blood and their ratio (CD4+/CD8+) were determined in 27 patients with IDC and in 20 healthy controls. NK cell percentage was also studied. Results: There were no significant differences between IDC and control groups with respect to T and B cell percentages. The percentages of CD4+ T cell subsets were similar in both groups (48.7±8.7 % vs. 43.5±9.7 % respectively; p=0.107). CD8+ T cell percentage was significantly decreased in patients with IDC than in controls (22.6±7.7 % vs. 28.2±8.2 %, respectively; p=0.044). CD4+/CD8+ ratio was markedly higher in patients with IDC than controls (2.6±1.8 vs. 1.6±0.6, respectively; p=0.006). There was no significant difference in the NK cell percentage between groups. Conclusion: Decreased CD8+ T cell subset is the cause of increased CD4+/CD8+ ratio, which may imply decreased self-tolerance and an immunoregulatory defect in the pathogenesis of IDC.