Background Provisional approach shows comparable or better outcomes compared with aggressive approach in bifurcation lesions. However, it is uncertain whether the side branch (SB) with substantial residual stenosis can be left untreated. Methods Between January 2004 and June 2006, 1691 bifurcation lesions (side branch diameter ���2.0 mm) in 1668 patients treated with drug-eluting stents (sirolimus-eluting stents or paclitaxel eluting-stents) were enrolled from 16 centers in Korea. Exclusion criteria were cardiogenic shock, ST-segment elevation acute myocardial infarction (MI) within 48 hours, and left main bifurcation lesions. Among these, we evaluated 1398 lesions (82.7%) in 1376 patients treated with 1-stent technique (main vessel stenting with or without kissing ballooning). Angiographic success was defined as achievement of TIMI 3 flow with a final residual stenosis <30% for the MV, and <50% for the SB. Medina classification (1,1,1), (1,0,1), or (0,1,1) were categorized as true bifurcation lesions. Results Angiographic success was achieved in 98.8% for the main vessel (MV) and 57.3% for the SB. The prevalence of true bifurcation was lower and final kissing ballooning was preformed more frequently in lesions with SB angiographic success than those without SB angiographic success. During follow up (median: 22 months), whether angiographic success of the SB was achieved or not was not significantly associated with major adverse cardiac events (MACEs: composite of cardiac death, MI, or target lesion revascularization) (6.8% versus 5.1%, p=0.20). Final kissing ballooning was independently predictive of MACEs [hazard ratio (HR) 2.16, 95% CI 1.34 to 3.48, p=0.002] with stent length in the MV (HR 1.02, 95% CI 1.002-1.03, p=0.02) and the use of paclitaxel-eluting stent (HR 1.99 versus sirolimus-eluting stent, 95% CI 1.28-3.10, p=0.002). Conclusions The rate of achieving the angiographic success of SB was relatively low, but the long-term outcomes were favorable regardless of the angiographic success of SB after 1-stent strategy. Ballooning of the SB would be performed cautiously when bifurcation lesion is treated by 1-stent technique.