Background: Previous studies have reported diffuse destabilization of atherosclerotic plaques in acute myocardial infarction (AMI). Objectives and Methods: We used pre-intervention intravascular ultrasound (IVUS) to assess 310 coronary culprit lesions (Group I: 125 lesions in ST segment elevation MI vs. Group II: 185 lesions in non-ST segment elevation MI). Culprit lesions were identified by a combination of electrocardiogram, wall motion abnormalities (ventriculogram or echocardiogram), and coronary angiogram. A ruptured plaque contained a cavity that communicated with the lumen with an overlying residual fibrous cap fragment. A lipid-pool like image was defined as a pooling of hypoechoic or echolucent material covered with a hyperechoic layer and a thrombus was a discrete intraluminal filling defects. Hypoechoic plaque was less bright compared with the reference adventitia. Results: Culprit lesions had larger external elastic membrane area (13.5짹4.9 mm2 vs. 11.9짹4.3 mm2, p=0.002), larger plaque plus media area (10.8짹4.4 mm2 vs. 9.1짹4.1 mm2, p=0.001) and plaque burden (78.7짹10.1% vs. 74.8짹12.0%, p=0.002), and culprit lesion site calcium arc was smaller (96짹90째 vs. 153짹114째, p=0.002) in Group I than in Group II. Culprit lesions were more predominantly hypoechoic in Group I than in Group II (62% vs. 40%, p<0.001). Culprit lesion plaque ruptures, lipid-pool like images, and thrombus were observed more frequently in Group I compared with Group II (46% vs. 29%, p=0.002, 39% vs. 25%, p=0.010, and 34% vs. 21%, p=0.006, respectively). Conclusions: Culprit lesions in ST segment elevation MI have more markers of plaque vulnerability (more plaque rupture and thrombus, and larger plaque mass) compared with lesions in non-ST segment elevation MI.