Anomalous origin of the left main coronary artery (LMCA) from the right sinus of the Valsalva or the proximal right coronary artery (RCA) is one of the most clinically important anomalies of coronary circulation. We report the case of a patient with chest pain and abnormal thallium myocardial perfusion scan in whom the anomaly was first detected on invasive coronary arteriography. The exact anatomic course of anomalous LMCA was confirmed using contrast enhanced computed tomography.