Background: Incomplete stent apposition (ISA) is known to occur at significant rates for various reasons after implantationof bare metal stent and drug eluting stent(DES). While the clinical repercussion of ISA have been thought to be insignificant, stent thrombosis have been suggested to have association ISA recently. However it is not well studied on the effect of ISA in left main coronary artery (LMCA), which may be more serious when the adverse event occurs. This study was performed to evaluate the long-term follow up results of the patients of ISA in LMCA. Method: Sixty-two patients who underwent coronary intervention and intravascular ultrasound after stenting at LMCA or the os of LAD/ LCx were enrolled in between May 2003 and January 2005. Twenty two patients were found to have incomplete apposition in LMCA(group1) and 40 patients were not (group2). Result: Baseline clinical characteristics were not significantly different between 2 groups. Type of DES (Cypher:Taxus, 86.4%:13.6%[group1], 75%:25%[group2]) stent diameter (3.18mm vs 3.23mm) and stent length (24.8mm vs 23.6mm), initial vessel area (23.3mm2 vs 22.2mm2, p>0.05), lumen area(14.4mm2 vs 11.8mm2, p>0.05) stent area(10.2mm2 vs 11.1mm2, p>0.05) were not significantly different between 2 groups. Follow up IVUS was done in 61% at 6 month-follow up (n=38). There were no significant differences in TLR and TVR at 18 months-follow up (TLR, 13.6% vs 7.5%, p>0.05; TVR, 4.5% vs 10%, p>0.05). There was no event of myocardial infarction in both groups, one unknown cause of death in each group. Conclusion: There was no significant difference in TLR and TVR between ISA group and non-ISA group of LMCA when followed for 18 months. The incompletely apposed DES in LMCA does not seem to impose clinical adverse effect.