Background: The full lesion coverage with longer stented segment has been regarded as the preferred strategy with drug-eluting stents (DES). However, the length of stented segment is known to be a traditional risk factor of stent thrombosis (ST). Methods: A total of 3,145 patients (4,667 lesions) who underwent DES (sirolimus-eluting stents; 2478 patients, 3603 lesions, and paclitaxel-eluting stents; 667 patients, 1064 lesions) from Feb, 2003 to Feb, 2006 were investigated. Using the logistic regression analysis and a receiver-operating characteristic (ROC) curve, we evaluated the association between DES thrombosis and stent length, and the cut-off value for the selection of the optimal stent length. Results: Mean stented number per lesion was 1.4짹0.6 and mean stented length per lesion was 34.2짹18.9 mm. During long-term follow up (29짹10 months), 68 patients (2.2%) developed ST. The overall rate of ST did not differ significantly between patients receiving sirolimus- or paclitaxel-eluting stents (2.0% vs. 2.7%; p=0.28). We found a significant relation between the rate of thrombosis and the stented length (Y= -4.563 + 0.050 X; odd ratio 1.1; 95% confidence interval [CI] for beta 1.01-1.20; R=0.71; p<0.01). The optimal cut-off value of stent length was 51.5 mm (area under ROC curve 0.613; 95% CI 0.54-0.69; p<0.01), with which the specificity and sensitivity were 84% and 38%. Conclusions: The risk of ST after DES implantation was significantly associated with the length of stented segment with the best discriminating value of 51.5mm.