Atresia of the left coronary artery ostium is extremely rare. We report the case of a 13-year-old boy who played volleyball in school and collapsed with severe chest pain during practice. He was referred to our hospital, and imaging modalities showed atresia of the left main coronary artery ostium. Urgent coronary ostioplasty was performed using a patch of 0.6% glutaraldehyde-treated autologous pericardium. His postoperative course was uneventful, and he has had a normal everyday life without chest pain 8 months postoperatively. Physicians should be aware of the patient's history, as in this case, because prompt imaging diagnosis is essential when there is a high likelihood that the event is related to myocardial ischemia. Since long-term outcome is uncertain even after successful surgical revascularization, close follow-up is required.