Previous studies have shown that transient no-reflow during coronary intervention but with Thrombosis in Myocardial Infarction (TIMI) grade 3 flow at the completion of the procedure is associated with increased in-hospital and 6-month mortality. We hypothesized that the use of intravascular ultrasound before intervention could identify morphologic features that were predictive of transient no-reflow in patients who had acute coronary syndrome (ACS). We analyzed 220 patients with ACS who had suitable intravascular ultrasound images that were acquired before intervention. We defined '' transient no-reflow '' as TIMI grade 0, 1, or 2 flow during the procedure and TIMI grade 3 flow at the completion of the procedure. We defined '' reflow '' as good coronary flow (TIMI grade 3 flow) during and after the procedure. Patients were categorized to a transient no-reflow group (n = 20) or a reflow group (n = 200). In the transient no-reflow group, vessel area and amount of plaque burden in the culprit lesion were significantly greater than in the reflow group (vessel 20.8 +/- 5.4 vs 16.4 +/- 6.2 mm(2), P < 0.01; plaque burden 0.90 +/- 0.03 vs 0.83 +/- 0.08, p < 0.001). The presence of ruptured plaque, lipid pool-like images, and thrombus formation were significantly higher in the transient no-reflow group than in the reflow group. Multivariate analysis identified the presence of thrombus formation (odds ratio 4.53, 95% confidence interval 1.03 to 20.0, p = 0.04) and larger plaque burden (odds ratio 1.79, 95% confidence interval 1.01 to 3.23, p = 0.05) as independent predictors of transient no-reflow. In conclusion, lesion morphologies are different for transient no-reflow and reflow. These findings suggest that the presence of thrombus formation and large plaque burden increase the risk for developing transient no-reflow during coronary intervention for ACS. (c) 200 Elsevier Inc. All rights reserved.