A new case of heart involvement in Lyme's disease is reported. Its originality lies in its strictly isolated nature, therefore revealing the disease; in its typical picture of myocardopericarditis combining, in various stages, infra-hissian conduction disorders, bouts of left ventricular insufficiency, pseudoischemic repolarisation disorders and finally a moderate pericardial shift; in the demonstration, during two successive heart explorations by NMR (Nuclear Magnetic Resonance), of a diffuse myocardial hypertrophy, subsiding in time, as the other symptoms of this patient.