A 13-year-old girl was admitted to our hospital with a history of syncope after exercise. Neither left ventricular (LV) function nor hypertrophy was detected by transthoracic echocardiography. However, 24-h Holter electrocardiogram demonstrated ST segment depression with increasing heartbeat. Exercise ²⁰¹Tl myocardial scintigram also demonstrated ischemia of the anterior LV wall. Multi-slice coronary computed tomography (CT) demonstrated hypoplasia of the left main coronary artery. The syncope on exertion was ascribed to myocardial ischemia due to hypoplasia of the left main coronary artery. We performed off-pump coronary artery bypass graft (CABG) (left internal thoracic artery-left descending artery). The postoperative course was uneventful and postoperative stress ²⁰¹Tl myocardial scintigram demonstrated the absence of myocardial ischemia. Coronary CT demonstrated good graft patency. To date, there has not been any recurrence of syncope on exertion. We herein report a successful off-pump CABG for a patient with syncope due to hypoplasia of the left main coronary artery. Syncope on exertion due to hypoplasia of the left main coronary artery is very rare. However, certain forms of congenital coronary anomalies are associated with adverse cardiac events, including sudden cardiac death. The diagnosis, therefore, can be important and CABG is indicated, especially when there is repetitive syncope due to myocardial ischemia.