Chest pain not related with injury is an important cause of visit to emergency departments; it accounts for 10 % of such visits and is caused by acute coronary syndrome. In most cases it is related to thrombus formation secondary to ulceration or rupture of an atherosclerotic plaque. Sometimes it is secondary to rare causes such as the presence of congenital coronary artery anomalies. We report the case of a patient with anomalous origin of the left main coronary artery, who was admitted to the hospital with a clinical picture suggestive of acute coronary syndrome without ST segment elevation. Its main aspects, clinical significance and therapeutic options are discussed.