A protective role of the presence of collateral arteries, generating smaller infarcts, improved ventricular function, fewer future cardiovascular events, and improved survival following a myocardial ischemia has been described in numerous reports. However little is known about atherosclerotic disease of the collateral vessels, and the possibility to treat critical stenosis of these vessels has never been described. Therefore this report describes a unique case of percutaneous coronary intervention on a well developed yet atherosclerotic coronary collateral vessel triggering an acute coronary syndrome with hemodynamic instability. In the present case balloon angioplasty and stenting of the collateral vessel was safe and effective. Nonetheless, further studies are warranted.