Background and Purpose: It is not well-known which jailed side branch(SB) needs additional complex procedures in bifurcation lesions. Moreover, morphologic assessment of the jailed SB lesions is relatively unreliable. We performed this study to evaluate the feasibility and clinical outcomes of the fractional flow reserve(FFR)-guided jailed SB intervention strategy in bifurcation lesions. Methods: Patients(pts) with jailed SB lesions(vessel size >2.0mm, % stenosis > 50%, lesion length < 10mm) after drug-eluting stent implantation at main branch(MB) were consecutively enrolled. Pts with left main bifurcation lesion and significant luminal narrowing at the parent vessel proximal to a bifurcation lesion or at the distal portion of SB were excluded. FFR was measured using a RADI 4 pressure wire at 5 mm distal and proximal to the ostium of the jailed SB. SB intervention was performed when SB FFR was <0.75. Follow-up(F/U) outcomes were assessed at 6 months after coronary intervention. Results: From 102 lesions initially enrolled, a total of 84 lesions(80 pts, male 61%, 61짹8 years) were finally included and analyzed. Eighteen lesions were excluded due to slow SB flow after stent implantation in 8, fail to recross SB in 3, protocol violation in 6 and AV block in 1. Mean % stenosis, reference vessel diameter and lesion length of the jailed SB was 78짹11%, 2.3짹0.3mm and 7.1짹3.1mm respectively. Mean FFR was 0.82짹0.11 at SB and 0.96짹0.04 at MB. Twenty out of 22 lesions with FFR<0.75 underwent kissing balloon inflation and SB-FFR was increased from 0.65짹0.07 to 0.86짹0.06(p<0.001). Among 64 pts eligible for 6 month f/u, f/u angiogram was performed in 50 and f/u FFR in 40 pts. During f/u, adjusted SB-FFR (SB-FFR/MB-FFR) was increased in SB lesions with no intervention(0.90짹0.06 to 0.91짹0.05, p=0.03). There were no changes in FFRs of MB and SB with intervention. Four cases of target lesion revascularization(6.3%) were occurred, 2 in MB, 1 in SB and 1 in both MB and SB. There was one non-cardiac death and no myocardial infarction. Conclusions: FFR-guided SB intervention strategy seems to be feasible and effective. This strategy may prevent unnecessary complex interventions and related complications