Background: Acute coronary syndrome (ACS) commonly results from vulnerable plaque rupture, and occasionally results from thrombus formation in lesions without plaque rupture. The aim of the present study was to clarify the clinical features of different etiology of ACS and clinical predictors of culprit plaque rupture assessed on intravascular ultrasound (IVUS). Methods and Results: One hundred and ten ACS patients with emergent coronary angiography were classified into 2 groups based on the presence or absence of culprit plaque rupture assessed on IVUS. Clinical characteristics were compared between the 2 groups. Culprit coronary plaque rupture was observed in 60 patients (55%). Patients with plaque rupture were younger and were more likely to be male (P<0.03 and P<0.02, respectively). In the rupture group, the prevalence of metabolic syndrome was higher (P<0.002), and among the components of metabolic syndrome, waist circumference was greater and serum high-density lipoprotein cholesterol level was lower (P<0.0001 and P=0.0004, respectively). IVUS-assessed lesion remodeling index was greater in the rupture group (P<0.0001). On multivariate analysis metabolic syndrome was an independent predictor of culprit plaque rupture (odds ratio =5.26, 95% confidence interval =1.49-21.40, P<0.02). Conclusions: Abdominal obesity and low high-density lipoprotein-cholesterol level are the characteristics of metabolic syndrome that seem to be the key factors for vulnerable plaque rupture with coronary compensatory enlargement.  (Circ J 2010; 74: 1936 - 1942)