Background: The mechanism of restenosis after stent implantation is principally neointimal hyperplasia (NIH). There is increasing evidence that white blood cells (WBCs), especially monocytes, play a central role in restenosis after stent implantation. This study assessed the relationship between pre-procedural peak monocyte count and NIH at 6-month follow-up 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. At scheduled 6-month follow-up, all patients received angiographic and intravascular ultrasound (IVUS) analysis. 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 than those in no 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.