An acute ischemic complication will develop in approximately 7% of patients undergoing coronary angioplasty. In more than half of these patients, thrombus, dissection, or both can be identified angiographically as the underlying cause of abrupt closure; in the remaining cases, an indeterminate morphology is observed. Predictors of dissection-mediated closure include degenerated vein graft, de novo stenosis, proximal tortuosity, high lesion grade, eccentricity, longer lesion length, and angulation. Thrombus-mediated closure is most apt to occur in degenerated vein grafts, lesions related to a recent myocardial infarction, and lesions that contain thrombus before angioplasty. However, the ability of these parameters to predict risk in an individual patient is rather weak. Angioscopy has revealed that the presence of yellow plaque confers a heightened risk of major complications. Lesions that contain areas of calcium adjacent to areas of soft plaque have been identified by intravascular ultrasound as a powerful predictor of major dissection. These sensitive new imaging techniques may eventually permit the targeting of more specific therapies to the underlying cause of closure.