Distal coronary embolism of thrombotic material is quite common in the setting of primary coronary angioplasty for evolving acute myocardial infarction. Embolization to another coronary artery is, however, much more uncommon. We report on a case in which a large thrombus migrated from the proximal left anterior descending artery (LAD) to the proximal left circumflex artery (CX) during inflation of the dilatation balloon. The putative mechanism was retrograde expulsion of the thrombus by the deploying balloon.