Background and Purpose: Jailed side branches after stent implantation in bifurcation lesions are usually assessed by quantitative coronary angiography (QCA). However, QCA does not always give the correct information in these complex lesions. We performed this study to evaluate the feasibility of physiologic assessment of jailed side branches with a pressure wire and to compare the measured fractional flow reserve (FFR) with the stenosis severity assessed by QCA. Methods: Seventy four patients (51 males, 61.5짹7.1 yo) with jailed side branches (vessel size >2.0mm, percent stenosis > 50% by visual estimation) after stent implantation in main branch 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 portion of the side branches were excluded. FFR was measured with a pressure wire (RADI 4, Uppsala, Sweden) at 5 mm distal and proximal to the ostial lesion of the jailed side branches. Bolus injection and continous infusion of adenosine were used to induce maximal hyperemia during FFR measurements. Results: Side branch FFR was successfully measured in 73 lesions (diagonal 57, obtuse marginal 13, posterior descending 2, posterolateral 1). There were no complications related to FFR measurement. Mean minimal luminal diameter, percent stenosis, reference vessel diameter and lesion length was 0.46짹0.29mm, 78.7짹13.1%, 2.2짹0.48mm and 7.6짹5.7mm respectively. Mean FFR at distal to the bifurcation was 0.84짹0.12 (range: 0.57-1) and at proximal to the bifurcation 0.96짹0.04 (range: 0.86-1.0). Among 73 lesions with > 50% stentosis by QCA, only 12 lesions showed FFR���0.75(TABLE). Conclusions: Physiologic assessment of jailed side branches with a pressure wire was safe and feasible. In physiologic point of view, QCA overestimates the stenosis severity of the jailed side branches after stent implantation.