Background: Whether side branch (SB) ballooning including kissing ballooning is mandatory or can improve clinical outcomes in 1-stent technique is uncertain. We sought to evaluate the impact of SB ballooning after main vessel (MV) stenting on clinical outcomes in coronary bifurcation lesions. Methods: Patients undergoing percutaneous coronary intervention for non-left main bifurcation lesions were enrolled from 16 centers between January 2004 and June 2006 (Coronary Bifurcation Stenting Registry). To assess the impact of SB ballooning after MV stenting in the provisional approach, we selected patients treated with 1-stent technique in 1 bifurcation lesion in the COBIS registry database. Bifurcation lesions with TIMI flow <3, or dissection of type B or worse after MV stenting were excluded. We compared major adverse cardiac events [MACE: cardiac death, myocardial infarction (MI), or target lesion revascularization (TLR)] between patients undergoing just MV stenting (group 1, n= 736) or SB ballooning after MV stenting (group 2, n=421). Results: During follow-up (median 22.2 months), group 2 had a higher incidence of MACE [36 (4.9%) in group 1, 37 (8.8%) in group 2, p=0.009] and TLR [25 (3.4%) in group 1, 32 (7.6%) in group 2, p=0.001], but not of cardiac death or MI (p=0.87). Multivariate analysis using Cox Hazard model showed that group 2 had a higher risk of MACE [HR 2.12; 95% CI 1.33-3.39; p=0.002) and TLR (HR 2.66; 95% CI 1.56-4.52; p<0.001). TLR was significantly higher in group 2 in MV (HR 2.18, 95% CI 1.27-3.76; p=0.004), but not in SB (p=0.94). There was no significant difference in the occurrences of stent thrombosis between the groups (0.5% in group 1 versus 1.0% in group 2, p=0.42). Conclusions: After MV stenting, SB ballooning may increase MACEs mainly due to increased TLR in patients treated with 1-stent technique for coronary bifurcation lesions.