Background Unheralded sudden death and acute myocardial infarction are common manifestations of coronary atherosclerosis. Such events are related to thrombotic occlusion at the site of non-flow limiting atheroscterotic plaques in epicardial coronary arteries. This study aimed to assess plaque characterization of nonculprit lesions in patients with acute coronary syndrome (ACS) compared with those with stable angina pectoris (SAP) determined by analysis of intravascular ultrasound (IVUS) radiofrequency (RF) data.Methods In 81 patients, nonculprit vessels with <50% diameter stenosis and nontarget segment of culprit vessels with <50% diameter stenosis were studied with IVUS. Tissue maps were reconstructed from RF data using IVUS-Virtual Histology software.Results Mean lipid core percentage was significantly higher in patients with ACS than in those with SAP ((25.78±6.30)% vs (9.11±4.90)%, P <0.001). In addition, patients with SAP showed more fibrotic vessels ((59.66±16.87)% vs (49.07±10.20)%, P <0.001). There was no significant difference in either mean calcium ((4.37±2.40)% vs (5.12±3.00)%, P=0.225) or fibrolipid ((24.94±9.40)% vs (25.82±13.60)%, P=0.731) percentages in nonculprit vessels, but the mean calcium percentage was significantly higher in nontarget lesions of culprit vessels ((5.51±3.29)% vs (3.57±2.10)%, P=0.003). In addition, there was a positive correlation between lipid core and remodeling index (RI) (r=0.847, P<0.001) and a negative correlation between fibrous tissue and RI (r=-0.946, P<0.001).Conclusions In this study, in both nonculprit vessels and nontarget lesion of culprit vessels, plaque characterization of nonculprit lesions determined by spectral analysis of IVUS RF data was significantly different in patients with ACS. The percentage of lipid core was significantly higher in patients with ACS than in those with SAP. Conversely, SAP patients showed more fibrotic content. In vivo plaque composition and morphological changes were related to remodeling of the coronary artery tree.