Background: From a clinical standpoint, a small vessel coronary disease was diagnosed by quantitative coronary angiography (QCA). Recently, intravascular ultrasound (IVUS) imagines have provided real vessel information compared with QCA. The aim of this study is to evaluate discrepancy of vessel information between IVUS and QCA before and after stent implantation in patients with small vessel coronary disease. Methods: From September 2001 to July 2003, 27 patients with 29 lesions in small vessels (<2.75 mm) and 113 patients with 131 lesions (���2.75 mm) underwent IVUS-guided stent implantation. Standard clinical , angiographic, and IVUS parameters were analyzed. We calculated the mean difference between IVUS and QCA vessel diameter (��IVUS-QCA_VD), minimal lumen diameter (MLD), and plaque area (PA) before and after stent implantation. We also evaluated correlation between (��IVUS-QCA_VD) and PA at the reference vessel site. Results: See below. Conclusion: In patients with small coronary vessel disease had a significantly larger ��IVUS-QCA vessel diameter, which had a stronger correlation with reference plaque area than large coronary vessel diseases. In addition, a tendency of larger ��IVUS-QCA minimal lumen diameter was observed at the stented segments in patients with small coronary vessel disease.