Background: Identification of subclinical high-risk plaques is potentially important because they may have greater likelihood of rupture and subsequent thrombosis. The purpose of this study was to assess the relationship between plaque composition determined by intravascular ultrasound(IVUS) radio frequency(RF) data analysis and clinical presentation. Methods: In 55 patients, a nonculprit vessel with< 50%diameter stenosis was studied with IVUS. Tissue maps were reconstructed from RF data using IVUS-Virtual Histology software. Results: Mean percentage of the different plaque components were 0.99%±0.9%, calcium; 68.04%±9.8%, fibrous; 19.31%±7.3%, fibrolipidic; and 9.43%±6.6%, lipid core. Mean lipid core percentage was significantly larger in patients with acute coronary syndrome(ACS) when compared with stable patients(12.26%±7.0%vs 7.40%±5.5%, P=.006). In addition, stable patients showed more fibrotic vessels(70.97%±9.3%vs 63.96%±9.1%, P=.007). There was no significant difference for either mean calcium(1.20%±1.1%vs 0.83%±0.7%, P=.124) or fibrolipidic(20.57%±6.9%vs 18.40%±7.6%, P=.281) percentages in ACS and stable patients, respectively. Vessel area obstruction did not differ between groups(46.49%±10.9%vs 42.83%±11.8%, P=.221). There was a significant, albeit weak, positive correlation between lipid core percentage and stenosis severity as determined by vessel area obstruction(r=0.34, P=.015). Conclusions: In this study, plaque characterization of nonculprit vessels using spectral analysis of IVUS RF data analysis was significantly related to clinical presentation. Percentage of lipid core, a feature related to acute coronary events and worse prognosis, was significantly larger in patients with ACS. Conversely, stable patients showed more fibrotic content. Background: Identification of subclinical high-risk plaques is potentially important because they may have greater likelihood of rupture and subsequent thrombosis. The purpose of this study was to assess the relationship between plaque composition determined by intravascular ultrasound (IVUS) radio frequency (RF) data Analysis and clinical presentation. Methods: In 55 patients, a nonculprit vessel with <50% diameter stenosis was studied with IVUS. Tissue maps were reconstructed from RF data using IVUS-Virtual Histology software. Results: Mean percentage of the different plaque components were 0.99 Mean lipid core percentage was significantly larger in patients with acute coronary syndrome (ACS) when% ± 0.9%, calcium; 68.04% ± 9.8%, fibrous; 19.31% ± 7.3%, fibrolipidic; and 9.43% ± 6.6% compared with stable patients (12.26% ± 7.0% vs 7.40% ± 5.5%, P = .006). In addition, stable patients showed more fibrotic vessels (70.97% ± 9.3% vs 63.96% ± 9.1%, P = There was no significant difference for either mean calcium (1.20% ± 1.1% vs 0.83% ± 0.7%, P = .124) or fibrolipidic (20.57% ± 6.9% vs 18.40% ± 7.6%, P = .281) percentages in ACS and stable patients, respectively . Vessel area obstruction did not differ between groups (46.49% ± 10.9% vs 42.83% ± 11.8%, P = .221). There was a significant, albeit weak, positive correlation between lipid core percentage and stenosis severity as determined by vessel area obstruction (r = 0.34, P = .015). Conclusions: In this study, plaque characterization of nonculprit vessels using spectral analysis of IVUS RF data analysis was significantly related to clinical presentation. Percentage of lipid core, a feature related to acute coronary events and worse prognosis, was significantly larger in patients with ACS. Conversely, stable patients showed more fibrotic content.