BACKGROUND AND OBJECTIVES: From a clinical standpoint, coronary artery disease in blood vessels measuring 2.5 mm to 2.75 mm, as accessed by quantitative coronary angiography (QCA), has been classified as a small vessel disease, and it is treated with percutaneous coronary intervention (PCI). The aim of this study was to evaluate the discrepancy of vessel size between intravascular ultrasound (IVUS) and QCA, and its late outcome before and after stent implantation in patients with small coronary artery disease (2.5-2.75 mm). SUBJECTS AND METHODS: We enrolled 135 patients having 143 lesions who underwent IVUS-guided PCI. Twenty-three patients (26 lesions) were in the small vessel (SV, < or =2.75 mm) group and 112 patients (128 lesions) were in the large vessel (LV, >2.75 mm) group. We evaluated the IVUS and QCA parameters' association with mortality, acute myocardial infarction (AMI) and target vessel revascularization (TVR) at the 1 year follow-up. RESULTS: On QCA, the pre-interventional reference vessel diameters and post-stent minimal lumen diameters in the SV group were smaller than those in the LV group. The discrepancy of vessel size between IVUS and QCA at the reference site was larger in the SV group than that in the LV group (1.44 mm vs. 0.92 mm, respectively p<0.05). This discrepancy was significantly associated with the plaque area in both groups (p<0.001). Despite of having larger stents implanted in the SV group than the LV group, there was no difference in mortality, AMI and TVR after 1 year between the 2 groups. CONCLUSION: A coronary artery disease measuring 2.5 mm to 2.75 mm by QCA revealed large vessels with a high percentage of plaque. The bigger stent implantation using IVUS did not show more complications after PCI and there were favorable clinical outcomes at 1 year for patients with this condition.