Background Angiographic evaluation of left main coronary artery (LMCA) bifurcation lesions is often limited.two dimensional (2D) quantitative coronary angiography (QCA) with segmental analysis provides accuracy for quantification of the degree of stenosis in the main vessel and side branch ostium but can be affected by foreshortening and variable magnification.The accuracy of three dimensional (3D) QCA has recently developed to overcome 2D QCA limitations,however,accuracy and precision of 3D bifurcation QCA measurements in LMCA bifurcation lesions has not been established.Methods We investigated whether such 3D and 2D bifurcation QCA measurements differ in their accuracy in assessing significant LMCA bifurcation lesions defined by intravascular ultrasound (IVUS) as a minimum luminal area (MLA) ＜6 mm2 of LMCA and MLA ＜4 mm2 of proximal left anterior descending (LAD) and/or proximal left circumflex (LCX) Results LMCA bifurcation lesions were assessed in 44 patients undergoing elective percutaneous coronary intervention.From 2D QCA measurements,MLA correlated moderately with threshold intravascular ultrasound MLA for LMCA (r=0.81,P ＜0.000 1),LAD (r=0.54,P=-0.000 1) and LCX (r=0.58,P ＜0.000 1).Severity of lesion as MLA by derived 3D QCA,correlated moderately with threshold intravascular ultrasound MLA for LMCA (r=0.84,P ＜0.000 1),LAD (r=0.53,P=-0.000 2); LCX (r=0.66,P ＜0.000 1).Overall,the C statistics tended to be slightly higher for 3D QCA and 2D QCA measurements in LMCA segment compared with proximal LAD and LCX segments,and there were no significant predictive power of percent diameter stenosis and percent area stenosis on 3D QCA for LCX IVUS MLA ＜4 mm2 (percent diameter stenosis:area under curve 0.55,cutoff 23％,sensitivity 88％,specificity 37％,P=-0.618 6; percent arer stenosis:area under curve 0.56,cutoff 41％,sensitivity 83％,specificity 38％,P=-0.518 4,respectively).Conclusions The accuracy of 3D bifurcation QCA in detecting significant LMCA bifurcation lesions is limited,especially the proximal LCX ostium.When IVUS is not available or contraindicated,3D QCA may assist in the evaluation of intermediate LMCA lesions with MLA.