Congenital atresia of the left coronary artery is a very rare condition. We report a case of a 13-year-old boy who was followed-up at our outpatient clinic from 6 months of age because of heart murmur. He had no symptoms for more than 10 years, and his left ventricular contraction was normal. Exercise electrocardiogram was performed at 7 and 10 years of age, and ST depression was not observed. He presented with chest pain and syncope on exertion at the ages of 11 and 12 years, respectively, and ST depression on exercise electrocardiogram was detected for the first time at 13 years of age. Cardiac echocardiography showed a high-choic antero-lateral papillary muscle of the mitral valve, prolapse of the mitral anterior leaflet, and mild mitral valve regurgitation. Coronary angiography did not demonstrate antegrade left coronary flow or dilation of the right coronary artery. We performed coronary artery bypass grafting using the left internal thoracic artery. Postoperatively, the ST depression resolved, and his exercise tolerability improved.