Background: Jailed side branches(JSB) after stent implantation are one of the most challenging lesions in interventional coronary angioplasty. Morphologic assessment of SB lesions is relatively unreliable. We performed this study to find out the pre-interventional angiographic predictors of functionally significant JSB lesions assessed by fractional flow reserve(FFR). Methods: Eighty–eight patients(92 lesions;55 males, 61.6짹8.7 years old) with JSB(vessel size>2.0mm, percent(%) stenosis > 50%, lesion length<10mm) after DES implantation were consecutively enrolled. Patients with regional wall motion abnormalities and significant luminal narrowing at the parent vessel proximal to a bifurcation lesion or at the distal part of the SB were excluded. FFR was measured with a pressure wire(RADI 4, Uppsala, Sweden) at 5 mm distal to the JSB lesions. Lesions with SB FFR<0.75 or which showed slow flow after main branch(MB) stent implantation were considered to have a functionally significant SB lesion. Results: Mean minimal luminal diameter, % stenosis, reference vessel diameter(RD) and lesion length of preintervention SB were 1.2짹0.5mm, 46짹19%, 2.3짹0.31mm and 6.8짹3.0mm respectively. Mean FFR at distal to the bifurcation was 0.81짹0.11(range:0.52-0.99) and at proximal to the bifurcation 0.96짹0.04(range:0.85-1.0). Thirty JSB lesions had functionally significant stenosis after MB intervention. The only angiographic predictor of functionally significant SB jailing before stenting was type 1 bifurcation lesion(odds ratio:5.22, 95% CI:1.31-20.79) in a multivariate analysis(covariates:bifurcation angle, plaque eccentricity, RD and % stenosis of MB and SB lesions). Only 14% of the other types of bifurcation lesion resulted in functionally significant SB jailing. The SB stenosis more than 80% after stenting was optimal cut off value(sensitivity:0.71, specificity:0.71) to predict functionally significant jailing, yielding an area under the curve of 0.72(95% CI 0.59-0.86). Conclusions: Type 1 bifurcation lesion was the predictor of functionally significant SB jailing. After MB intervention, only lesions with high grade stenosis seem to require an additional procedure.