AbstractFifty‐eight patients with symptomatic congestive heart failure were examined for T‐lymphocyte subsets in the peripheral blood using two‐color laser flow cytometry as a noninvasive diagnostic procedure. The final diagnoses established by catheterization and endomyocardial biopsy were dilated cardiomyopathy (DCM, n=24), myocarditis (MC) by the Dallas criteria (n=12), and coronary heart disease (CHD, n=16). The CD8+CD11‐ (cytotoxic T) subset was significantly low in patients with DCM (13.9±4.4 vs. controls, p<0.05) in comparison with MC (20.7 + 10.9) and CHD (22.3±5.9). Moreover, the CD4+2H4+ (suppressor/inducer T) subsets were higher in patients with DCM (27.3±6.9 vs. controls, p<0.01) than in those with MC (17.3±7.8) and CHD (15.6+7.9). The CD4/CD8 and CD4+2H4+/CD8+CD11‐ ratios were examined and compared with those of normal controls (NC n=16). The CD4+2H4+/CD8+CD11‐ ratio was clearly higher in patients with DCM (2.2+0.9 vs. controls, p<0.01) than in those with MC (1.1±0.6) CHD (0.9±0.7). A CD4+2H4+/CD8+CD11‐ ratio of > 1.6 was considered to facilitate diagnosis of dilated cardiomyopathy with 79% sensitivity and 70% specificity. There was no significant increase in the ratios between MC and CHD. However, the proportion of the CD8+Leu7+ (natural suppressor) subset of circulating T lymphocytes in patients with MC was statistically higher (19.1±6.3% vs. controls, p<0.05) than in DCM or CHD. An elevated ratio of CD4+2H4+/CD8+CD11‐ among peripheral blood lymphocytes may thus be a useful marker for differential diagnosis of dilated chronic cardiomyopathy from myocarditis and coronary heart disease.