Background: It is well known that sirolimus-eluting stent (SES) has a considerably lower optimal minimal stent area (MSA) threshold compared to bare metal stent (BMS). In SIRIUS IVUS substudy, the absolute optimal stent expansion (OSE) of DES was presented as ���5.0mm2. However, in the real field, interventional cardiologists frequently met the MSA below 5.0mm2 after successful stent implantation. Objective: The aim of this study is to clarify the appropriateness of the absolute IVUS criteria suggested by SIRIUS trial in SES implantation. Method: 88 patients (96 lesions) with serial coronary angiography (baseline and 6-9 month follow-up) were enrolled in this study. The IVUS was performed after SES deployment. The patients were divided to two groups according to an absolute IVUS criteria, defined as minimal stent area (MSA) ���5.0mm2: group 1 ���5.0mm2, group 2 <5.0mm2. An angiographic restenosis rate and major adverse cardiac events were evaluated. Results: See as a following table. Conclusions: Even though there was no statistical significance, OSE in SIRIUS IVUS substudy looks valuable parameter. However, when the reference vessel diameter is smaller then 3.0mm, the outcomes were not significantly different whether OSE was achieved or not. Therefore, the concept of OSE in the era of DES should be reformed.