Objective: The objective is to assess endothelial function 6 months after drug-eluting stents(DES) implantation compared with bare metal stents(BMS) implantation. Methods: Patients who had BMS or DES implantation in left anterior descending artery(LAD) and showed no restenosis in follow up angiography were classified to BMS group(5 patients) and DES group(8 of Cypher, 7 of TAXUS). Endothelium-dependent vasomotion was determined by measuring the change of diameter after intracoronary injection of 20ug-acetylcholine(Ach) at 2cm proximal to stent using a microcatheter. Endothelium-independent vasomotion was assessed after an intracoronary bolus of 2mg-nitrates. The measuring was done at 5 points; proximal and distal reference, proximal and distal edge of stent, and stented segment by quantitative coronary angiography. Intravascular ultrasound was done to check dissection or atherosclerosis in measuring points and patients with such findings were excluded. Results: In stented segments, no significant vasomotion by Ach or nitrates was observed in both groups. However, in distal edge and distal reference DES group showed significant vasoconstriction compared with BMS group (-26.5%vs-0.6%;p=0.01, -23.5%vs-0.9%;p=0.01). Cypher group and TAXUS group were each compared with BMS group and both showed significant vasoconstriction by Ach in distal edge and distal reference (Cypher: -27.6%vs-0.6%;p=0.01, -20.8%vs-0.9%;p=0.02, TAXUS: -25.4%vs-0.6%;p=0.04, -26.6%vs-0.9%;p=0.01). Comparing the change of diameter between proximal edge and distal edge and between proximal reference and distal reference after Ach injection, BMS group showed no difference (p=0.1, 0.25) while DES group showed a significant decrease of diameter in distal portion compared to proximal portion (Cypher: p=0.001, p=0.002, TAXUS: p=0.03, p=0.001). There was no difference in vasodilatation by nitrates between two groups. Conclusions: The DES group shows the impairment of endothelium-dependent vasodilatation in the distal portion of the stent. However, endothelium-independent vasodilatory response to nitrates is preserved. The results suggest that implantation of DES could be associated with drug-induced endothelial dysfunction.