An acute Lyme carditis affects about 0.3-4% of patients with Lyme borreliosis. The acute period of the disease may be associated with critical atrioventricular conduction abnormalities (complete heart block), supraventricular and ventricular arrhythmias as well a left ventricular failure. Normally, Lyme carditis is completely reversible. Therefore the prognosis largely depends on the management of the acute complications and early antibiotic therapy. Even if the symptoms are spontaneously reversible, antibiotic therapy should be applied to prevent a chronic cardiomyopathy and other manifestations of Lyme borreliosis. We report on a 47-year old patient with acute ECG changes initially suggesting an acute coronary syndrome. However, case history and the erythema migrans indicated an acute Lyme carditis which was confirmed serologically and by myocardial biopsy later.