Background Left internal thoracic artery (LITA) grafts have superior patency to saphenous vein grafts (SVG). Because shear stress augments the release of nitric oxide throughout the LITA endothelium, shear stress and shear rate in coronary artery bypass grafts (CABG) may play an important role in the higher patency, so the aim of the present study was to evaluate and compare the rheologic parameters in CABG using LITA and SVG. Methods and Results Rheologic examinations were done in 197 patients using a vacuum-suction glass tube viscometer after CABG surgery was completed. Shear stress and shear rate were calculated from the geometry of the graft, blood flow in the graft and blood viscosity. Of 197 patients, 177 underwent LITA grafting to the left anterior descending artery (LAD) and 160 had SVG anastomosis to coronary arteries. Mean wall shear stress in the LITA grafts to the LAD (13.8±1.0 dyne/cm²) was nearly 4-6-fold larger than that in the SVG grafts. Mean shear rate (559.1±57.0 s^-1) of LITA-LAD grafts was approximately 2-3-fold higher than that of SVG. Conclusion These results suggest that high wall shear stress and shear rate play an important role in the higher patency rate of LITA grafts. (Circ J 2005; 69: 700 - 706)