Background- Attenuated plaques are unusual intravascular ultrasound (IVUS) findings in patients with acute coronary syndrome; their significance is not fully understood. Methods and Results- We reviewed clinical presentation and angiographic and pre-intervention IVUS findings in 293 patients with acute coronary syndrome undergoing percutaneous coronary intervention (PCI) without a distal protection device: 184 with non-ST elevation myocardial infarction (STEMI) and 109 with STEMI. Attenuated plaque (defined as deep echo attenuation without calcification) was observed in 81 patients (27.6%): 43.1% of STEMI, 18.5% of non-STEMI (p<.001). Overall, the level of C-reactive protein (CRP), lesion site plaque burden, and remodeling index were significantly greater; lesion site luminal area was significantly smaller; and angiographic thrombus, initial coronary flow < TIMI 2, IVUS target lesion lobulated mass, positive remodeling, plaque rupture, and no reflow after PCI were more common in lesions with attenuated plaque (Table). There were no significant differences in cardiac risk factors; use of thrombolytics, angiomax, or GPIIbIIIa inhibitors; or peak levels of cardiac enzyme after PCI comparing the two groups. Conclusion: Attenuated plaque was more common in patients with STEMI than non-STEMI. Attenuated plaque was associated with a higher CRP level, more severe and complex lesion morphology, reduced coronary blood flow before PCI, and especially no-reflow after PCI.