Dilated cardiomyopathy (DCM) continues to be an aetiologically unknown heart muscle disease. Recent clinical and experimental data have suggested a temporal relationship with viral myocarditis. The clinical diagnosis of a chronic myocarditis is unspecific. The evaluation of endomyocardial biopsies by light microscopy and their histological classification according to the Dallas criteria is limited by the difficulty in differentiating and quantifying infiltrating lymphocytes from non-inflammatory interstitial cells. Using immunohistological methods that allow better identification and quantification of infiltrating lymphocytes and which provide further evidence for an activated immunological process within the myocardium, myocarditis was diagnosed on endomyocardial biopsy in 48 of 130 patients (37%). According to both haemodynamic and immunohistological findings, 31 of these patients were allocated for immunosuppressive treatment. After a 6 month treatment period with 6-methylprednisolone, 23 patients reported an improvement according to the NYHA classification. Lymphocytic infiltrations were abolished by corticoid treatment in 24 patients. Left ventricular systolic function was improved in 20 patients (64%) as indicated by an increased ejection fraction and stroke volume with a concomitant decrease of left ventricular end diastolic pressure. Our study suggests that immunosuppressive treatment in a subgroup of patients with dilated cardiomyopathy who have a continuing active immunohistologically proven inflammatory process results in a clinical, haemodynamic and immunohistological improvement in 60-70% of patients.