Intraoperative measurements have suggested an interrelation between the diameter of aortocoronary venous bypass (ACVB) grafts and the hemodynamic properties that may influence the functional results and long-term graft patency. We therefore studied the function of 35 nonobstructed ACVB grafts in 34 patients 1-59 months after bypass grafting. The mean blood velocity and flow were assessed using the roentgen videodensitometric technique. Graft diameters were obtained by morphometry from 35- or 70-mm cineangiograms taken in two orthogonal projections. The regional resistance of the graft-dependent area was calculated as mean aortic pressure/ACVB flow. The average graft diameter was 3.67 mm, the average graft-to-host vessel diameter ratio 1.5, the average ACVB blood velocity 8.5 cm/sec, the average flow 55 ml/min, and the average regional resistance 2.8 mm Hg/ml/min. No significant differences were observed for different graft locations or for different postoperative time intervals. Neither ACVB blood velocity nor flow correlated convincingly with graft size, but there was a significant inverse relation between graft diameter and the resistance of the graft-dependent area. We conclude that ACVB hemodynamics generally cannot be predicted by the dimensions of the graft in the postoperative state. Our data suggest that venous bypass grafts may adjust in size to the needs of the myocardial region supplied by the graft after operation.