Cardiac involvement occurring early in Borrelia burgdorferi infection is a clinical manifestation of human Lyme disease. Therefore, two patients with acute complete atrioventricular heart blocks and unexplained recurrent dizziness were studied. Both patients had significantly elevated serum titers of IgM and IgG antibodies to B. burgdorferi. Right ventricular subendocardial biopsies showed dense infiltrates consisting of lymphocytes and plasma cells. Silver staining revealed spirochetes characteristic of B. burgdorferi near and in the infiltrates, between the muscle fibers, and in the endocardium. One patient responded to penicillin; the other did not, necessitating installation of a pacemaker. Thus, permanent heart damage may result from cardiac involvement in Lyme disease.