A four-year-old boy presented to the emergency room with exercise-induced syncope. He had experienced four prior episodes of syncope, all of which occurred after playing vigorously. Loss of consciousness lasted approximately 2–4 min, and was associated with pallor, followed by spontaneous recovery. The baseline electrocardiogram was normal. An echocardiogram revealed a structurally normal heart, with a prominent right coronary artery. The origin of the left coronary artery could not be visualized clearly. There was no evidence of supravalvar aortic stenosis, or a history of Kawasaki syndrome. The patient demonstrated no ischemic changes on treadmill testing with a Bruce protocol. During cardiac catheterization, selective right coronary angiography (Fig. 1a) revealed a dilated right coronary artery (arrow), with retrograde filling of the circumflex and left anterior descending (arrowhead) branches, and atresia of the main stem of the left coronary artery.¹ The patient underwent bypass grafting of the left anterior descending branch, using the left internal thoracic artery, and was discharged home 6 days after the operation. Post operative angiography (Fig. 1b) confirmed patency of the internal thoracic arterial graft, with flow into the left anterior descending branch and retrograde flow into the circumflex branch.