Background: Zotarolimus-eluting stent (ZES: Endeavor^TM) have been known to carry a lower risk of stent thrombosis due to rapid re-endothelialization. Objective: We compared the coronary endothelial dysfunction in patients with a Zotarolimus- versus Sirolimus-eluting stent (SES: Cypher^TM) implantation at 6 months follow-up. Methods: In 40 patients (male 21, 54.7짹9.5 yrs) treated with IVUS guided intervention using single stent to the mid segment of left anterior descending artery (15 ZES, 15 SES, 10 BMS: bare metal stent), endothelial function was prospectively estimated at preintervention and postintervention 6 months follow-up, by incremental acetylcholine (Ach1 :10, Ach2: 20, Ach3: 50, Ach4: 100 µg/min) and nitrate (200 µg/min) infusion into the left coronary ostium. The vascular response was quantitatively measured in the segment 5 mm proximal and distal to the stent. All anti-anginal agents were withheld for at least 72 hours prior to coronary angiography. Results: At 6 months follow-up, significant vasoconstriction to Ach was observed in both ZES and SES groups (ZES: <0.05, SES: p<0.01). There was a lesser vasoconstriction to Ach in ZES group than in SES group (Figure, p<0.05). Conclusions: Vasoconstriction in response to Ach in the peristent region was less pronounced in ZES group than SES group at 6 months follow-up, which suggests that endothelial dysfunction associated with ZES can be restored more rapidly than those of SES.