An anomalous left coronary artery arising from the right sinus of Valsalva with a trajectory between the aorta and the main pulmonary artery is a rare isolated congenital anomaly carrying a high risk of sudden death and of acute myocardial ischaemia, especially in children and young adults in a context of exercise. It is usually a post-mortem finding. The authors report the rare case of a 12 year old child who suffered acute myocardial infarction on exercise due to this condition. The diagnosis was made in the acute phase by echocardiography. Surgical correction was performed as this is the only means of prevention of sudden death of an ischaemic recurrence. Effort syndromes in children or young adults should lead to a request for echocardiographic examination to exclude the diagnosis in a non-invasive manner as well as those of hypertrophic cardiomyopathy or congenital aortic stenosis.