Background: Provisional side branch (SB) intervention strategy is still preferred for bifurcation lesions even in the era of drug-eluting stents (DES). We performed this study to evaluate the feasibility and functional outcomes of fractional flow reserve (FFR)–guided jailed SB intervention strategy. Methods: Patients with jailed SB lesions (vessel diameter>2mm, lesiong length<10mm) after DES implantation at the main branch (MB) were consecutively enrolled. FFR was measured at 5 mm distal and proximal to the ostial lesion of the jailed SB. SB intervention was performed using a smaller balloon than the SB vessel diameter only if the SB FFR was <0.75. Follow-up FFR was measured at 6 months after coronary intervention. Adjusted SB FFR (SB-FFRadj = [SB FFR]/[MB FFR]) was used to exclude the influence of MB stenosis. Results: A total of 124 lesions in 120 patients were enrolled and FFR was measured in 95 lesions (LAD; 72%, type 1; 41%, cypher; 68%) in 91 patients (male 68%, 62 years). Post-stent SB/MB percent stenosis, reference diameter and lesion length were 78짹10/11짹17%, 2.3짹0.3/2.9짹0.3mm and 6.8짹3.0/28짹10mm, respectively. SB FFR was 0.81짹0.12 and MB FFR, 0.96짹0.04. Only 38% of lesions >75% stenosis had FFR<0.75. Kissing balloon inflation (KB) was performed in 26 lesions out of 29 lesions with FFR<0.75 (balloon artery ratio: 0.8짹0.1). FFR was increased to 0.85짹0.06 from 0.65짹0.08 after KB and FFR>0.75 was achieved in 24 lesions (92%). Follow-up coronary angiography and FFR measurement were done in 84% and 83% of eligible patients. Only 5 lesions(8%) had functional restenosis (FFR<0.75). There was no significant FFR change(���) in all, KB and non-KB patients during follow-up. There was no difference in ���SB-FFRadj between KB and non-KB groups(-0.02짹0.10 vs. 0.01짹0.04). Taxus stent had more ���SB-FFRadj than Cypher stent (0.02짹0.05 vs. -0.03짹0.09, p=0.02). Use of TAXUS stent was the single independent predictor for ���SB-FFRadj. Conclusion: FFR-guided SB intervention with KB using a relatively small-sized balloon was feasible and effective in maintaining the functional patency of SB.