The paper presents the information on the involvement of the cardiovascular system in the patients with Lyme's disease as well as the results of serological investigation in patients with myocarditis and dilatation cardiomyopathy for the presence of antibodies to the causative agent of Lyme's disease with the aid of enzyme immunoassay. Out of 71 sera 3 specimens had the values of optical density higher than the upper normal limits. Based on the investigation conducted and the analysis of the clinical picture, the authors suspected a Borrelia origin of these cardiac involvements. However, to verify the diagnosis morphological examination of endomyocardiac biopsy specimens was found to be mandatory.