Pathologic studies have been suggested that atherosclerotic components are often included within thrombi in infarct related arteries (IRA). Virtual Histology-Intravascular Ultrasound (VH-IVUS) misclassifies thrombus as either fibrotic or fibrofatty plaque. Conversely, the predictive accuracy of VH-IVUS detection of dense calcium (Ca) is high. Methods. We used VH-IVUS to assess the presence of Ca within intraluminal thrombi in 50 pts with acute ST elevation myocardial infarction (STEMI). VH-TCFAs were defined as necrotic core (NC) >10% of plaque area, plaque burden >40%, and NC in contact with the lumen for ���3 image slices. Remodeling index (RI) was lesion/reference external elastic membrane >1.05. Results. Using VH-IVUS imaging, thrombi were divided into 2 groups (Table): Ca-containing intraluminal thrombi (18 pts) and thrombi without Ca (32 pts). Calcified intraluminal thrombi were located in 9 LAD, 2 LCX, and 7 RCA. Vessel size, lesion length, plaque burden, minimal lumen area, and positive remodeling were similar in the two groups (data not shown). However, a VH-TCFA was present adjacent to 43% (14/32) of Ca-thrombi vs 22% (4/18) of thrombi without Ca (p=0.063). Importantly, post-PCI peak troponin was higher in the setting of Ca-thrombi (551.56짹 589.97 ng/ml) vs 251.18짹273.36 ng/ml in thrombi without Ca, p=0.027. Conclusion. Ca is present within 40% of IRA thrombi in pts with STEMI. These Ca-containing thrombi appear to be at increased risk of distal embolization during PCI.