Background: It has been suggested that sirolimus-eluting stents (SES) provoke a more sustained inflammatory response (IR) in neointimal hyperplasia (NIH). The purpose of this study was to compare morphological vessel characteristics, including post-stent IR in NIH, between patients with SES and bare metal stents (BMS) using optical coherence tomography (OCT). Methods and Results: Thirty-seven patients underwent OCT at their post-stent follow-up. OCT signal-intensity deviation (normalized standard-deviation; OCT-NSD) values in NIH were compared between the 2 groups. In addition, the serum concentration of high-sensitivity C-reactive protein (hs-CRP) was measured. Stent-malapposition rate (1.78% vs. 0.7%; P=0.016), uncovered stent-strut rate (16% vs. 3.7%; P=0.0002), peri-stent ulcer like appearance (PSUA; 50% vs. 0%; P=0.006) were all significantly higher in the SES group than in the BMS group, respectively. The OCT-NSD value was also significantly higher in the SES group than in the BMS group (0.213±0.005 vs. 0.198±0.005; P<0.001), as was the hs-CRP level (2.54±1.89 vs. 0.64±0.3mg/L; P=0.0006). In addition, a significant positive correlation was found between hs-CRP and OCT-NSD (r=0.471; P=0.0025). Conclusions: PSUA-morphology was specific in the SES group, and higher levels of OCT-NSD and hs-CRP after SES implantation suggest sustained IR in NIH compared with following BMS implantation. These different characteristics may be some of the background that promotes thrombus formation as a late-stage post-stent complication of SES. (Circ J 2011; 75: 1633-1640)