To investigate the mechanism of cardiac dysfunction in myocarditis, myoglobin, an intracellular oxygen-transport, was immunohistochemically examined in biopsy specimens obtained from the right side of the ventricular septum and left ventricular free wall in 58 patients with myocarditis and 19 controls. Sections 4 microns thick were stained by the indirect immunoperoxidase method using a polyclonal antibody to human myoglobin as the primary antibody. Under light microscopy, the intensity of myoglobin immunoreactivity in the tissue section was semiquantitatively classified from grade 0 to grade 3. Then, the grade of myoglobin staining was compared with clinical, hemodynamic and histopathologic parameters. In right and left ventricular specimens, the grade of myoglobin staining was positively correlated with ejection fraction, but inversely with left ventricular end-diastolic and end-systolic volume indices. The percentage of myocytes with grade 0 was correlated with the number of mononuclear cells in the specimens. In addition, the grade of myoglobin staining in right ventricular specimens was positively correlated with the duration of illness but inversely correlated with the number of mononuclear cells. In 4 patients who had serial biopsies, the ejection fraction was improved and the grade of myoglobin staining was increased in the convalescent stage. These results indicate that myoglobin staining reflects the intensity of myocarditis and a decrease of myoglobin may be important as one of the pathogenetic factors of cardiac dysfunction in myocarditis.