Lyme borreliosis (LB) is a multisystem disorder that may cause self-limiting or chronic diseases of the skin, the nervous system, the joints, heart and other organs. The aetiological agent is the recently discovered Borrelia burgdorferi. In 1980, cardiac manifestations of LB were first described, including acute conduction disorders, atrioventricular block, transient left ventricular dysfunction and even cardiomegaly. Pathohistological examination showed spirochaetes in cases of acute perimyocarditis. Recently, we were able to cultivate Borrelia burgdorferi from the myocardium of a patient with long-standing dilated cardiomyopathy. In this study, we have examined 54 consecutive patients suffering from chronic heart failure for antibodies to Borrelia burgdorferi. On ELISA, 32.7% were clearly seropositive. The endomyocardial biopsy of another patient also revealed spirochaetes in the myocardium by a modified Steiner's silver stain technique. These findings give further evidence that LB is associated with chronic heart muscle disease.