Human cytomegalovirus (CMV) can persist in many organs after primary infection. Not only is it suspected to cause morbidity during reactivation in patients under immunosuppression, but it may also induce long-term latency by chronic disease, e.g. in the myocardium. Endomyocardial biopsies of 27 patients with active myocarditis, 35 patients with healing, 41 patients with healed and 25 patients with ongoing myocarditis according to the Dallas Criteria and 52 patients with dilated cardiomyopathy (DCM) and the biopsies of 25 healthy heart donors were studied for persisting CMV-DNA by polymerase chain reaction (PCR) and in-situ hybridization (ISH). CMV-DNA could be assessed in 5-14% of patients in the different stages of myocarditis and in 22% of patients with DCM. Although two biopsies of the control group showed a positive result, studies by in-situ hybridization demonstrated that in patients with heart muscle disease CMV persists in all cell types including myocytes, whereas in the controls it is only found in interstitial cells. CMV antigens could not be detected in the myocardium with our methods. It must be assumed that infection by CMV is more a persistent or latent than an active infection.