Background: The mechanism of restenosis after stent implantation is principally neointimal hyperplasia (NIH). Despite the widespread use of intracoronary stent, in-stent restenosis (ISR) remains a major clinical problem. Objectives: This study assessed the relation between pre-procedural peak monocyte count and NIH after successful stent implantation. Methods: We performed coronary stent implantation in 85 patients (85 de novo lesions). Peripheral blood sample was obtained in all patients before coronary angiography for measurement of peripheral monocyte. All patients received angiographic and intravascular ultrasound (IVUS) follow-up at six months after stenting. Results: The circulating monocyte count was significantly larger in ISR group than that in no ISR group (654짹62/mm3 vs 461짹222/mm3, p<0.001) and was significantly larger in re-intervention group than that in no re-intervention group (660짹72/mm3 vs. 470짹216/mm3, p<0.001). On IVUS study, the lesion site lumen area was significantly smaller and lesion site plaque plus media cross sectional area (CSA) was significantly larger and pre-interventional remodeling index was significantly higher in ISR group. A significant positive correlation was found between pre-interventional peak monocyte count and pre-interventional P&M CSA and follow-up NIH area (r=0.311, p=0.007, r=0.465, p<0.001, respectively). But, no significant correlation between pre-interventional peak monocyte count and pre-interventional remodeling index (r=0.134, p=0.255). Conclusion: Our results suggest that circulating monocytes play a principal role in the process of in-stent NIH after successful stent implantation.