This case report is on a 69-year old male patient treated with cephalosporins because of suspected myocarditis due to borreliosis. Using transthoracic echocardiography a big aneurysm of the proximal part of the left coronary artery was detected. Coronary angiography revealed an aneurysm 1.2 cm in diameter at the origin of the left anterior descending branch and confirmed the initial diagnosis. In addition, coronary three-vessel disease with reduced left ventricular function was found. Coronaritis due to Lyme borreliosis could not be ruled out with certainty. The patient was relatively asymptomatic, and hence conservative therapy was recommended. The case described here serves as a basis for a discussion on the aetiology, clinical manifestation, diagnosis and therapeutic management of coronary aneurysms.