Background: Previous studies have shown that renal insufficiency is an independent predictor of significant coronary artery disease and is associated with subclinical atherosclerosis. The aim of this study was to assess plaque morphology and morphometry in acute myocardial infarction (AMI) patients with varying degrees of renal dysfunction using intravascular ultrasound (IVUS). Methods: A total of 72 patients who underwent IVUS imaging of an infarct-related artery before percutaneous coronary intervention were grouped according to calculated creatinine clearance (CrCl): Group I [CrCl >70 ml/min (n=31)]; Group II [CrCl 30 to 69 ml/min (n=30)]; Group III [CrCl <30 ml/min, (n=11)]. Results: CrCl was 96짹15 ml/min in Group I, 53짹9 ml/min in Group II, and 14짹5 ml/min in Group III. Group III was oldest (56짹13 vs. 67짹13 vs. 71짹13 years, p<0.001 compared Group I with III) and had more diabetes (39% vs. 45% vs. 54%, p=0.033). Although no significant differences were found in lesion site external elastic membrane cross-sectional area (CSA), minimum lumen CSA, minimum lumen diameter, plaque burden, and remodeling pattern among three groups, Group III had largest reference segment plaque plus media CSA (2.8짹1.4 vs. 4.9짹2.4 vs. 5.6짹3.2 mm2, p=0.023 compared Group I with III) and largest reference segment plaque burden (27짹11 vs. 36짹12 vs. 42짹13%, p=0.032 compared Group I with III). Culprit lesion plaque rupture and multiple plaque rupture were observed most frequently in Group III (32.3% vs. 43.3% vs. 63.6%, p=0.005, and 6.4% vs. 16.7% vs. 27.3%, p=0.002). Maximal plaque cavity CSA (0.98짹0.29 vs. 1.29짹0.67 vs. 1.53짹0.59 mm2, p=0.001 compared Group I with III), total plaque cavity CSA (1.80짹0.92 vs. 2.62짹1.42 vs. 3.45짹1.78 mm2, p<0.001 compared Group I with III) were greatest, and plaque rupture length (2.2짹0.9 vs. 2.5짹1.4 vs. 2.8짹1.3 mm, p=0.043 compared Group I with III) was longest in Group III. Conclusion: Decreased renal function was associated with more severe reference segment disease and with unstable plaque morphology. Our findings show that renal dysfunction is related with more unstable lesion with diffuse atherosclerosis in patients with AMI.