Introduction: Silorimus and paclitaxel have many different properties. Sirolimus inhibits G1 cell cycle, but paclitaxel inhibits M-phase. Moreover, they have different diffusion capacity and distribution in the vascular wall. Accordingly sirolimus eluting stent (SES, Cypher) and paclitaxel eluting stent (PES, Taxus) have revealed different rate of angiographic restenosis and target lesion revascularization. Hypothesis: Optical coherence tomography (OCT) could detect a thin layer of neointimal hyperplasia(NIH) and malapposition with higher resolution(<10關m) than intravascular ultrasound. Therefore, we have tried to reveal the differences of SES and PES in NIH thickness, stent exposure, malapposition and instent microthrombi at 9 months after implantation using OCT. Method: Motorized OCT pullback (1 mm/s) was performed in consecutively implanted 25 SESs (25 patients, 11 with acute coronary syndrome(ACS) and 14 with non-ACS) and 21 PESs (21 patients, 11 with ACS and 10 with non-ACS). NIH thickness inside each strut and percent NIH area in each cross section were measured. Results: In total, 9309 struts in 1092-mm single-stented segments were analyzed. Overall, NIH thickness and percent NIH area were 133.3 짹 162.7 mm and 15.7 짹 13.2 %, respectively. Overall rates of exposed struts and exposed struts with malapposition were 6.0 % and 2.0 %, respectively. There were significant differences in NIH thickness(86.6짹73.0 關m for SESs, 187.5짹213.1 關m for PESs. p < 0.001) and percent NIH area(11.1짹7.5 % for SESs, 20.5짹16.0 % for PESs. p < 0.001) between two other stent groups. Rates of exposed struts and exposed struts with malapposition were 7.6 % and 2.4 % for SESs, 3.5% and 1.4% for PESs (p < 0.247 and p< 0.579, respectively). Instent microthrombi were observed in 52% of SESs and in 14.3% of PESs (P< 0.007) Conclusion: In conclusion, SESs revealed significantly thinner NIH thickness and smaller NIH area than PESs. These findings suggest the better efficacy of SESs at preventing in-stent restenosis than PESs.