Dilated cardiomyopathy (DCM) is characterized by progressive reduction in contractile function and by dilatation of the right and left ventricles. Abnormalities of the cellular and humoral immune system are present in patients with myocarditis and DCM. Several antibodies against cardiac structures have been detected in DCM patients. The functional significance of cardiac autoantibodies is under debate. For certain antibodies, in vitro data indicate a negative effect on cardiac performance. Furthermore, recent data have provided evidence that cardiac antibodies themselves induce DCM. Cardiac antibodies belong to the IgG fraction and can be eliminated by immunoadsorption therapy. Recent clinical studies showed that removal of antibodies by immunoadsorption improves cardiac function of patients with DCM, indicating that activation of the humoral immune system plays a functional role in DCM.