The use of sequential coronary artery bypass grafts has been advocated because of improved graft runoff and increased blood flow through the graft with this technique. To examine the influence of runoff, quantitated in terms of coronary vascular resistance, on the velocity of blood flow through coronary artery bypass grafts, we made two sets of simultaneous pressure and flow measurements in 106 single grafts and in 35 double sequential grafts. The first set of measurements was obtained following the aortic anastomosis and the second set of measurements was made following completion of the coronary anastomosis. Resistance of the coronary bed was calculated from the two sets of measurements. The velocity of blood flow through the grafts was calculated from vein graft diameter and the second flow measurement. No significant difference was found between the diameter of single vein grafts (4.0 +/- 0.05 mm.) and sequential vein grafts (4.1 +/- 0.09 nm.). Coronary vascular resistance in the sequential grafts (100.0 +/- 15.6 RU) was lower than that in single grafts (174.6 +/- 14.6 RU, p = 0.001). Velocity of blood flow through the proximal segment of the sequential grafts (11.1 +/- 1.1 cm/sec) was higher than that through single grafts (7.5 +/- 0.6 cm/sec, p = 0.003). The proximal segment of the sequential bypass graft has a higher velocity of blood flow than that seen in a single bypass graft. To obtain maximum hemodynamic advantage with the possibility of improving long-term patency rates, it is advisable to use the smaller coronary artery for the proximal sequential anastomosis.