This study investigated the distribution and phenotype of mononuclear cells in endomyocardial biopsy (EMB) specimens from patients with viral myocarditis (VMC, 25 cases) and dilated cardiomyopathy (DCM, 10 cases). T lymphocytes, including Th/i cells in VMC and Tc/s cells in DCM, predominated in the infiltrating mononuclear cells in these specimens. The morphological relationship between infiltration of T lymphocytes and pathological changes of myocardia was also observed and neither macrophages nor B lymphocytes were found.