Lyme's disease is a multi-system condition due to infection with a spirochete (Borrelia Burgdorferi), transmitted by a tick. Cardiac involvement, which is not systematic, usually presents with transient atrioventricular block of varying degree. The authors describe an unusual presentation of the cardiac involvement of Lyme's disease with chest pain resembling an acute coronary syndrome in a 32 year old man. The characteristic skin lesion (erythema migrans), the positivity of IgM serology, the myocardial scintigraphic results and the negativity of the work-up of other causes of this pain led to a diagnosis of myocarditis, the outcome of which was favourable with treatment by amoxycillin (3 g/day, orally).