Objectives: Slow flow phenomenon is a serious complication of percutaneous coronary intervention (PCI) and associated with poor prognosis. We sought to evaluate the characteristics of lesions predisposing to slow/no-reflow phenomenon during primary PCI in patients presenting with acute myocardial infarction. Methods: The study subjects consisted of 57 consecutive patients (mean 58.5짹14.5 years old, 45 males) who underwent primary PCI for acute myocardial infarction and intravascular ultrasound-virtual histology (IVUS-VH) examination. Slow flow was defined as  thrombolysis in myocardial infarction grade 2 after PCI. Results: Slow flow was developed in 12 patients (8 males). Patients with slow flow was likely to be older (67.5짹13.8 yrs old vs. 56.2짹13.9 yrs old, p=0.015), had more cardiogenic shock (16.7% vs. 2.2%, p=0.046), larger fibrofatty volume over the entire lesion length (36.7짹25.5mm³ vs. 18.0짹18.6mm³, p=0.006), higher remodeling index (1.10짹0.17 vs. 0.99짹0.16, p=0.043), larger plaque area (16.2짹5.4mm² vs. 12.5짹4.9mm², p=0.025), fibrous area(8.0짹3.3mm² vs. 5.4짹3.0mm², p=0.014), and fibrofatty area (2.7짹2.2mm² vs. 1.3짹1.6mm², p=0.016) at the minimal lumen site than those without slow flow (37 males). Multivariate analysis revealed that the fibrofatty volume over the entire lesion length was the only independent factor (beta=0.359, 95% confidence interval 0.002 to 0.012, p=0.006) for slow flow during primary PCI. Conclusions: This study suggests that slow flow may be dependent on the tissue characterization (fibrofatty volume) of the underlying lesion at the time of the primary PCI for acute myocardial infarction.