Background: Although the internal thoracic artery was proven superior to saphenous vein graft in long-term patency, it is thought to be a more resistive conduit than the vein graft. Moreover, patency studies comparing both left and right internal thoracic arteries have provided results favoring the former. Fractional flow reserve is an established functional index of coronary blood flow. Methods: To compare the fractional flow reserve between both internal thoracic arteries and saphenous vein grafts, 43 bypass grafts were studied 6 months after revascularization. Intra-graft pressures were measured during cardiac catheterization using a pressure-wire advanced to the first distal anastomosis of 12 left internal thoracic arteries (ITAs), 10 right ITAs and of 21 vein grafts. Pressure gradients between the aorta and the graft were measured at baseline and during a maximal hyperemia. Results: At baseline, pressure gradient was recorded in the left ITA (2.9 ± 2.2 mmHg), in the right ITA (1.2 ± 1.2 mmHg) and in the vein graft (0.4 ± 0.7 mmHg). During maximal hyperemia, pressure gradient increased to 9.6 ± 3.2 mmHg in left ITA, to 4.5 ± 2.0 mmHg in the right ITA (p < 0.001 vs left ITA) and to 3.3 ± 2.7 mmHg in vein (p < 0.001 vs left ITA; NS vs right ITA). Fractional flow reserve was 0.90 ± 0.04 in left ITA, 0.95 ± 0.03 in right ITA (p < 0.01 vs left ITA) and 0.96 ± 0.03 in vein (p < 0.001 vs left ITA). Conclusion: Internal thoracic arteries and saphenous vein grafts allow myocardial revascularization with minimal resistance to maximal blood flow. The resistance appears significantly higher in left ITA compared to both the right ITA and venous grafts.