BACKGROUND The risk of side branch compromise has been reduced using guidewire protection. However, the clinical outcomes of patients with or without the major side branch ballooning after the parent vessel stenting have not been verified. We sought to assess the effect of major side branch ballooning after the parent vessel stenting on six months clinical outcomes. METHODS The major side branch (defined as > 1.5mm in diameter) originating from the parent vessel lesion showed 50% to 90% index and post-PCI diameter stenosis with TIMI 3 flow. Patients in the side branch ballooning (SBB) group (n=101) and no side branch ballooning (NSBB) group (n=69) were retrospectively enrolled after stenting the parent vessel lesion (n=170) from March 2003 to June 2005. All major adverse cardiac events (MACEs) were determined for 180 days after drug-eluting stent implantation in parent vessel with or without major side branch ballooning. RESULTS The more patients in the NSBB group were smokers compared with the SBB group [37.6% (n=38) vs. 15.9% (n=11), p=0.002]. Baseline and post-PCI angiographic findings showed no significant differences between the 2 groups. The numbers of patients with left anterior descending lesions were similar between the 2 groups [76.8% (n=43) in the SBB group vs. 70.1% (n=61) in the NSSB group, p=0.618]. Complex lesion such as type B2 or C comprised more than 80% of patients in both groups (82.2% in the SBB group vs. 88.3% in the NSBB group, p=0.446). Six months MACEs were similar between the 2 groups (Table). CONCLUSION The major side branches with TIMI 3 flow after parent vessel stenting might have no differences on six months clinical outcomes with or without sequential side branch ballooning.