Embolization of thrombotic material during primary angioplasty may occur even if the procedure is performed with appropriate medication as well as in hospital centers having a high volume of procedures. Over recent years the introduction of mechanical devices for thromboaspiration and/or distal protection have reduced the incidence of embolization thus improving myocardial reperfusion and survival. We report the case of a large thrombus migrated from the proximal left circumflex artery to the proximal left anterior descending artery during balloon inflation. In our opinion, the cause was retrograde expulsion of the thrombus by the deploying balloon.