Since 1980 cardiac manifestations of Lyme borreliosis have been described as selflimited conduction and transient left ventricular disorders or even cardiomegaly. An etiologic role of Borrelia burgdorferi in long-standing chronic heart disturbances is suggested by the cultivation of a strain of Borrelia burgdorferi which we were able to isolate from an endomyocardial biopsy of a patient with long-standing dilated cardiomyopathy. The aim of this study was to acquire information about the prevalence of serum antibodies to Borrelia burgdorferi in patients with dilated cardiomyopathy. By ELISA, we studied the sera of 72 consecutive patients with chronic heart failure due to dilated cardiomyopathy, of 55 patients with coronary heart disease, and of 61 healthy blood donors; positive ELISA values were determined in 26.4%, 12.7%, and 8.2% of serum samples, respectively. These findings further suggest an association or even an etiologic role for Borrelia burgdorferi in dilated cardiomyopathy.