We describe a rare case of post-infarction myocardial rupture leading to communication between the left ventricle and coronary sinus, which eventually led to a left-to-right shunt. The observation was made in an elderly woman with subacute infero-posterior myocardial infarction. Diagnosis was initially made by transthoracic echocardiography (elicited by the finding of high-velocity flow within a dilated coronary sinus), and was confirmed in greater detail at cardiac magnetic resonance. This description adds to the list of known post-infarction mechanical complications. The finding of high-velocity flow within the coronary sinus after myocardial infarction suggests the possibility of myocardial rupture leading to left ventricle to coronary sinus communication; an unusual but potentially treatable complication.