Background: Plaque rupture and subsequent thrombus formation is the most important mechanism leading to an acute myocardial infarction (AMI). Previous pathological study showed diabetic patients had a larger content of lipid-rich atheroma and macrophage infiltration compared with nondiabetic patients. This is consistent with a greater probability of coronary plaque rupture in diabetic patients. However, data on the intravascular ultrasound (IVUS) findings in diabetic patients with AMI with plaque rupture are lacking. Objectives and Methods: The aim of this study was to assess the impact of diabetes mellitus on IVUS findings in 112 AMI patients (58 ST segment elevation and 54 non-ST segment elevation MI) with plaque ruptures. IVUS findings included ruptured plaque (a cavity that communicated with the lumen with an overlying residual fibrous cap fragment), multiple ruptured plaques (separated by a >5-mm length of artery containing smooth lumen contours), and a thrombus (discrete intraluminal filling defects). Results: Baseline high-sensitivity C-reactive protein (4.6짹4.6 mg/dl vs. 2.4짹4.2 mg/dl, p=0.050) and triglyceride levels (158짹84 mg/dl vs. 127짹52 mg/dl, p=0.041) were significantly higher in diabetic patients compared with non-diabetic patients. Reference segment plaque burden was greater in diabetic patients compared with non-diabetic patients (37짹10% vs. 31짹12%, p=0.006). The presence of multiple plaque ruptures (60% vs. 29%, p=0.001) and thrombus (72% vs. 52%, p=0.032) were more common in diabetic patients compared with non-diabetic patients. Plaque cavity was significantly larger (2.6짹1.6 mm2 vs. 2.2짹1.2 mm2, p=0.046) and ruptured plaque length was significantly longer (3.0짹1.6 mm vs. 2.5짹1.3 mm, p=0.031) in diabetic patients compared with non-diabetic patients. Conclusions: Diabetic AMI patients with IVUS-evident plaque ruptures have more plaque vulnerability (more frequent multiple plaque ruptures and thrombus) accompanied with higher inflammatory status compared with non-diabetic AMI patients with plaque ruptures.