Background: Distal embolization during primary percutaneous coronary interventions(PCIs) may affect myocardial reperfusion. We evaluated the prevalence and features of embolization during primary PCI and its relationship with clinical and angiographic variables. Methods: Forty-six consecutive patients with acute myocardial infarction underwent primary PCI with a filter-based distal protection device. Histopathologic analysis was performed on retrieved embolic fragments, assessing the presence and relative amount of fibrin, necrosis, lipid droplets, collagen, mucopolysaccharides, and leukocytes, as well as the total debris volume. Such variables were related to baseline clinical and angiographic variables. Results: Embolic material was recovered in 41(89%) of 46 cases, with a mean total debris volume of 1.2±2.2 mm3. Prevalent histopathologic patterns were organized thrombus(47%), fresh thrombus(29%), and plaque fragments(24%). At multivariate analysis, none of the baseline clinical variables considered significantly predicted the total debris volume. Among angiographic variables, angiographic signs of high thrombus burden(cut-off coronary occlusion pattern or large intracoronary minus image) independently predicted the total debris volume at multivariate analysis(odds ratio 15.8, P< .005). Compared with its nonuse, abciximab did not affect the total number and the mean total volume of embolized material(15±16 vs 10±8 fragments, 1.5±2.5 vs 1.0±1.9 mm3, respectively, for both P >.20), or its qualitative composition. Conclusions: Distal embolization occurs in most patients during primary PCI and mainly consists of plaque fragments and partially organized thrombi, which are likely to be scarcely responsive to antiplatelet drugs. Baseline angiographic signs of a high thrombus burden are the only significant predictors of the extent of distal embolization. Background: Distal embolization during primary percutaneous coronary interventions (PCIs) may affect myocardial reperfusion. We evaluated the prevalence and features of embolization during primary PCI and its relationship with clinical and angiographic variables. Methods: Forty-six consecutive patients with acute myocardial infarction underwent primary PCI with a filter-based distal protection device. Histopathologic analysis was performed on retrieved embolic fragments, assessing the presence and relative amount of fibrin, necrosis, lipid droplets, collagen, mucopolysaccharides, and leukocytes, as well as the total debris volume. Such variables were related to baseline clinical and angiographic variables. Results: Embolic material was recovered in 41 (89%) of 46 cases with a mean total debris volume of 1.2 ± 2.2 mm3. Prevalent histopathology patterns were organized thrombus (47%), fresh thrombus (29%), and plaque fragments (24%). At multivariate analysis, none of the baseline clinical var Among angiographic variables, angiographic signs of high thrombus burden (cut-off coronary occlusion pattern or large intracoronary minus image) independently predicted the total debris volume at multivariate analysis (odds ratio 15.8, P <.005 ). Compared to its nonuse, abciximab did not affect the total number and the mean total volume of embolized material (15 ± 16 vs 10 ± 8 fragments, 1.5 ± 2.5 vs 1.0 ± 1.9 mm3, respectively, for both P> .20) , or its qualitative composition. Conclusions: Distal embolization occurs in most patients during primary PCI and mainly consists of plaque fragments and partially organized thrombi, which are likely to be scarcely responsive to antiplatelet drugs. Baseline angiographic signs of a high thrombus burden are the only significant predictors of the extent of distal embolization.